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Columbia  ©ntoertfttp 

College  of  logicians:  ano  burgeons 
Htbrarp 


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in  2010  with  funding  from 

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^-TEXT-BOOK 


OF 


Normal  Histology: 


INCLUDING 


AN  ACCOUNT  OF  THE  DEVELOPMENT 

OF  THE  TISSUES  AND  OF 

THE  ORGANS. 


BY 

GEORGE  A.  PIERSOL,  M.D., 

PROFESSOR   OF   ANATOMY   IN    THE   UNIVERSITY   OF   PENNSYLVANIA. 


WITH  FOUR   HUNDRED   AND  NINE  ILLUSTRATIONS,   OF    WHICH    THREE 

HUNDRED  AND  FIFTY-EIGHT  ARE  FROM  ORIGINAL 

DRA  WINGS  B  Y   THE  A  UTHOR. 


FOURTH  EDITION. 


PHILADELPHIA'. 

J.  B.  LIPPINCOTT   COMPANY, 

189s. 


Copyright,  1893, 

BY 

J.  B.  Lippincott  Company. 


?(o 


Printed  by  J.  B.  Lippincott  Company.  Philadelphia 


PREFACE   TO   FOURTH   EDITION. 


The  favorable  reception  accorded  the  ' '  Histology' '  has  necessi- 
tated the  printing  of  a  fourth  edition  so  soon  after  the  first  appear- 
ance of  the  book  that  but  few  changes  have  been  made  at  the  present 
time  ;  these  consist,  for  the  most  part,  of  slight  alterations  of  the 
text  and  the  illustrations. 

The  opinions  expressed  by  those  most  capable  of  passing  judgment 

lead  the  author  to  hope  that  the  preparation  of  the  work  has  not 

been  without  gain  to  both  teacher  and  student. 

G.  A.  P. 

October  i,  1895. 


PREFACE. 


In  the  preparation  of  these  pages  the  aim  of  the  author  has  been 
to  present  descriptions  which  should  include  the  salient  features  of 
the  various  structures  with  sufficient  fulness  to  impress  important 
details  without  wearying  minutiae  :  many  years  of  teaching  have  con- 
vincingly shown  that  too  great  conciseness  of  statement,  on  the  one 
hand,  and  too  great  elaboration  of  detail,  on  the  other,  are  alike 
unsatisfactory  to  the  student  in  his  efforts  to  gain  an  adequate  and 
lasting  knowledge  of  minute  anatomy. 

The  recognition  of  the  underlying  morphological  relations  of  the 
tissues  alone  can  bring  the  appreciation  of  the  broad  principles 
requisite  for  the  elevation  of  histology  from  a  maze  of  barren  details 
to  a  study  full  of  interest  and  suggestion.  In  order  that  these  wider 
bearings  may  become  apparent,  a  brief  account  of  the  embryological 
processes  and  the  histological  differentiation  concerned  in  the  de- 
velopment of  the  tissues  and  the  organs  has  been  added  to  the 
descriptions  of  the  adult  structures.  The  desirability  of  keeping  the 
size  and  scope  of  the  volume  within  the  limits  adapted  to  its  primary 
purpose  of  text-book  has  forbidden  the  systematic  consideration  of 
embryological  data,  and  much  of  interest  relating  to  the  earlier 
stages  of  development  has  been  necessarily  omitted. 

In  adopting  the  character  of  the  illustrations  choice  has  been 
influenced  by  the  reflection  that  the  mission  of  such  drawings  is 
instruction,  and  that  the  illustrations  best  accomplishing  that  end 
are  of  most  value  for  the  object  at  hand.  With  the  exception  of 
those  taken  from  other,  duly  acknowledged  sources,  the  drawings 
have  been  made  by  the  author  in  nearly  all  cases  with  the  aid  of  the 
camera  lucida  or  from  photo-micrographs.  While  sufficiently  dia- 
grammatic to  be  efficient  aids  in  the  comprehension  of  the  text,  the 
drawings  are  faithful  likenesses  of  the  original  preparations ;  the 
latter  as  far  as  possible  have  been  taken  from  human  tissues. 


vj  PREFACE. 

For  manifest  reasons,  references  and  bibliography  have  been 
omitted,  except  in  connection  with  statements  where  mention  of  the 
name  of  the  authority  has  seemed  desirable.  The  author  wishes  to 
express  his  obligation  to  the  writings  of  Kolliker,  Ranvier,  Schwalbe, 
Waldeyer,  Retzius,  Stohr,  Flemming,  O.  Hertwig,  Schaefer,  Golgi, 
Ramon  y  Cajal,  and  others,  as  well  as  to  many  papers  found  in  the 
Archiv  fur  mikroskopische  Anatomie  and  other  journals. 

G.  A.  P. 

University  of  Pennsylvania, 

Philadelphia,  September  30,  1893. 


CONTENTS. 


CHAPTER    I. 

PAGES 

The  Cell  and  the  Tissues 11-25 

The  elementary  tissues  ; 

The  typical  cell ; 

Protoplasm — arrangement  and  structure ; 

The  nucleus  and  the  nucleolus  ; 

The  paranucleus ; 

The  centrosome  and  the  attraction-spheres  ; 

Vital  manifestations  of  the  cell ; 

Direct  cell-division ; 

Indirect  cell-division — karyokinesis  ; 

Maturation  and  fecundation  of  the  ovum  ; 

Segmentation  of  the  ovum  ; 

The  tissues — cellular  and  intercellular  constituents  ; 

The  blastodermic  layers  and  their  derivatives. 

CHAPTER    II. 

The  Epithelial  Tissues 26-34 

Varieties  of  epithelium ; 
Squamous  epithelium  ; 
Columnar  epithelium  ; 
Modified  epithelium ; 
Glandular  epithelium ; 
Neuro-epithelium  ; 
Endothelium  ; 

Development  of  epithelium  ; 
Development  of  endothelium. 

CHAPTER    III. 

The  Connective  Tissues 35-57 

Forms  of  connective  tissue  ; 

Cellular  elements  ; 

Intercellular  constituents— white  fibrous  and   yellow  elastic 

tissue  ; 
Mucoid  tissue ; 
Tendon ; 
Elastic  tissue  ; 

Development  of  fibrous  and  elastic  tissue  ; 
Adipose  tissue ; 
Hyaline  cartilage  ; 
Elastic  cartilage ; 
Fibro-cartilage  ; 
Development  of  cartilage  ; 
Bone — spongy  and  compact ; 
Structure  of  compact  bone  ; 

vii 


-j  CONTENTS. 

PAGES 

The  periosteum  ; 

The  marrow  of  bone  ; 

Development  of  bone ; 

Endochondral  bone  ; 

Periosteal  bone ; 

Summary  of  bone-development. 

CHAPTER    IV. 

The  Muscular  Tissues ...     58-68 

Non-striated  muscle— distribution  and  structure  ; 

Striated  muscle-structure ; 

Arrangement  of  muscle-fibres ; 

Cardiac  muscle  ; 

Development  of  muscular  tissue. 

CHAPTER    V. 

The  Nervous  Tissues 69-82 

Nerve-cells  ; 

Nerve-fibres — structure ; 
Medullated  nerve-fibres  ; 
Non-medullated  nerve-fibres ; 
Nerve-trunks — structure ; 
Supporting  tissues  of  nerve-centres  ; 
Ganglia — structure ; 
Development  of  nervous  tissues. 

CHAPTER    VI. 

The  Peripheral  Nerve-Endings 83-93 

Terminations  of  sensory  nerves  ; 
Special  sensory  nerve-endings ; 
Tactile  cells ; 
Tactile  corpuscles ; 
End-bulbs  ; 

Nerve-endings  in  non-striated  muscle  ; 
Nerve-endings  in  striated  muscle  ; 
Nerve-endings  in  tendon ; 
Nerve-endings  in  blood-vessels ; 
Nerve-endings  in  glands ; 
Neuro-epithelium  of  the  sense-organs. 

CHAPTER    VII. 

The  Circulatory  System 94~JI4 

The  arteries  ; 

The  veins ; 

The  capillary  blood-vessels  ; 

The  heart ; 

Development  of  the  blood-vessels  ; 

Development  of  the  heart ; 

The  blood  ; 

The  colorless  blood-cells  ; 

The  colored  blood-cells ; 

Effect  of  reagents  on  human  blood  ; 

Blood-crystals  ; 

Development  of  the  blood-corpuscles. 


CONTEXTS. 


CHAPTER    VIII. 

PAGES 

The  Lymphatic  System 1 15-135 

The  lymphatic  spaces ; 

The  lymphatic  vessels ; 

The  lymphatic  tissues  ; 

Simple  lymph-follicles ; 

Compound  lymph-glands  ; 

The  spleen ; 

The  thymus  body  ; 

The  serous  membranes  ; 

The  synovial  membranes  ; 

Development  of  the  lymphatic  system  ; 

Development  of  the  spleen  ; 

Development  of  the  thymus  body. 

CHAPTER    IX. 

Mucous  Membranes  and  Glands 136-143 

Mucous  membranes — structure  ; 

Glands — varieties ; 

Tubular  glands ; 

Saccular  glands  ; 

Glandular  epithelium ; 

Glandular  ducts ; 

Mucous  glands  ; 

Serous  glands  ; 

Changes  due  to  functional  activity  ; 

Development  of  glands. 

CHAPTER    X. 

The  Digestive  Tract 144-190 

The  oral  cavity ; 

The  teeth  ; 

Development  of  the  teeth  ; 

The  tongue ; 

The  papillae  of  the  tongue  ; 

The  salivary  corpuscles  ; 

The  tonsils  ; 

The  pharynx  ; 

The  oesophagus ; 

The  stomach  ; 

The  glands  of  the  stomach  ; 

The  intestines ; 

The  intestinal  villi ; 

The  intestinal  glands  ; 

The  glands  of  Lieberkiihn  ; 

The  glands  of  Brunner  ; 

The  solitary  glands  ; 

The  agminated  glands,  or  Peyer's  patches  ; 

The  liver ; 

The  gall-bladder  ; 

The  accessor}-  digestive  glands  ; 

The  parotid  gland ; 


x  CONTENTS. 

PAGES 

The  submaxillary  gland  ; 

The  sublingual  gland  ; 

The  pancreas  ; 

Development  of  the  digestive  tract ; 

Development  of  the  accessory  digestive  glands. 

C  H  APTER    XI. 

The  Urinary  Organs 191-206 

The  kidney  ; 

The  renal  sinus  and  the  ureter ; 

The  urinary  bladder  ; 

The  urethra  ; 

Development  of  the  urinary  organs. 

CHAPTER    XII. 

The  Male  Reproductive  Organs 207-223 

The  testicle ; 

Spermatogenesis ; 

The  epididymis  ; 

The  semen ; 

The  penis  ; 

The  prostate  gland ; 

The  glands  of  Cowper. 

CHAPTER    XIII. 

The  Female  Reproductive  Organs 224-245 

The  ovary  ; 

The  ovum  ; 

The  escape  of  the  ovum  ; 

The  parovarium  ; 

The  paroophoron  ; 

The  oviduct ; 

The  uterus ; 

The  vagina ; 

The  genitalia ; 

The  glands  of  Bartholin  ; 

The  mammary  glands  ; 

Milk; 

Development  of  the  reproductive  organs. 

CHAPTER    XIV. 

The  Respiratory  Organs 246  260 

The  larynx  ; 

The  trachea  ; 

The  bronchi ; 

The  lungs  ; 

The  pleura  ; 

The  thyroid  body  ; 

Development  of  the  respiratory  organs  ; 

Development  of  the  thyroid  body. 


CONTENTS.  xi 


CHAPTER    XV. 

PAGES 

The  Skin  and  its  Appendages 261-281 

The  skin  ; 

The  nails  ; 

The  hair ; 

The  sebaceous  glands  ; 

The  sweat-glands ; 

Development  of  the  skin  and  its  appendages. 

CHAPTER    XVI. 

The  Central  Nervous  System 282-335 

The  membranes  of  brain  and  cord  ; 

The  spinal  cord  ; 

The  medulla ; 

The  pons ; 

The  crura  cerebri  ; 

The  cerebellum  ; 

The  cerebral  cortex ; 

The  hippocampus  major ; 

Tne  fascia  dentata ; 

The  fimbria  ; 

The  septum  lucidum ; 

The  corpus  striatum  ; 

The  optic  thalamus  ; 

The  corpora  quadrigemina  ; 

The  olfactory  lobe ; 

The  white  matter  of  the  cerebrum  ; 

The  pituitary  body  ; 

The  pineal  body ; 

The  suprarenal  body  ; 

Development  of  the  nervous  tissues. 

CHAPTER    XVII. 

The  Eye  and  its  Appendages        „   ,    .   .    336-376 

The  cornea  ; 

The  sclera ; 

The  choroid ; 

The  ciliary  body  ; 

The  iris  ; 

The  irido-corneal  angle  ; 

The  retina  ; 

The  optic  nerve  and  entrance  ; 

The  crystalline  lens ; 

The  vitreous  body ; 

The  blood-vessels  of  the  eye  ; 

The  lymphatics  of  the  eye  ; 

The  nerves  of  the  eye  ; 

The  eyelids  ; 

The  Meibomian  glands  ; 

The  conjunctiva ; 

The  lachrymal  apparatus  ; 

The  capsule  of  Tenon  ; 

Development  of  the  eye. 


xjj  CONTENTS. 


CHAPTER    XVIII. 

PAGES 

The  Organ  of  Hearing 377~4oi 

The  external  ear ; 

The  tympanic  membrane  ; 

The  middle  ear; 

The  ear-ossicles  ; 

The  Eustachian  tube ; 

The  internal  ear  ; 

The  saccule  and  the  utricle  ; 

The  semicircular  canals  ; 

The  cochlea ; 

The  ductus  cochlearis ; 

The  scala  vestibuli ; 

The  scala  tympani ; 

The  ductus  endolymphaticus ; 

The  development  of  the  ear. 

CHAPTER    XIX. 

The  Nasal  Mucous  Membrane 402-406 

The  respiratory  region ; 
The  olfactory  region  ; 
Development  of  the  nasal  fossae. 

APPENDIX. 

The  Most  Useful  Histological  Methods 407-429 

Fixation  of  the  tissue  ; 

Fixation  reagents ; 

Preservation  of  the  tissue  ; 

Staining ; 

Staining  solutions ; 

Embedding ; 

Interstitial  embedding ; 

Paraffin  method ; 

Celloidin  method ; 

Section-cutting  ; 

Cutting  ribbon-series ; 

Fixing  sections  to  the  slide  ; 

Mounting  sections ; 

Finishing,  labelling,  and  storing  slides  ; 

Outline  of  standard  method  ; 

Weigert's  staining  method  ; 

Golgi's  silver  method  ; 

Golgi's  gold  method  ; 

Silver  staining ; 

Staining  chromatin  filaments ; 

Injecting  capillary  blood-vessels. 

Index 43r 


LIST  OF   ILLUSTRATIONS. 


FIG.  PAGE 

i.  Colorless  blood-cell n 

2.  Typical  cell — ovum  of  cat 12 

3.  Structure  of  the  cell 13 

4.  Cells  exhibiting  the  paranucleus  (Plainer) 14 

5.  Segmenting  ova  of  ascaris  megalocephala  (Boveri) 14 

6.  Direct  cell-division  of  colorless  blood-corpuscle 15 

7.  Karyokinesis — diagram  of  close  skein  ( Rabl- Schiefferdecker) 16 

8.  Karyokinesis — diagram  of  loose  skein  1  Rabl- Schiefferdecker) 16 

9.  Karyokinesis — diagram  of  polar  field  (Rabl- Schiefferdecker) 17 

10.  Karyokinesis—  diagram  of  migration  of  segments  (Rabl ) 18 

11.  Karyokinesis — epidermal  cells  from  larva  of  newt 19 

12.  Segmenting  ova,  showing  centrosomes  and  attraction-spheres  (Boveri)    .  20 

13.  Large  marrow-cell  with  multiple  nuclei 20 

14.  Maturation  and  fecundation  of  ovum  ( O.  Hertwig) 22 

15.  Blastodermic  layers  of  rabbit  embryo 24 

16.  Squamous  epithelium 28 

17.  Stratified  squamous  epithelium  in  section 28 

18.  Isolated  cells  of  stratified  squamous  epithelium 28 

19.  Prickle-cells  from  epidermis 29 

20.  Simple  columnar  epithelium 29 

21.  Stratified  columnar  epithelium 29 

22.  Ciliated  epithelium     . 30 

23.  Isolated  elements  of  ciliated  epithelium 30 

24.  Goblet-cells 31 

25.  Pigmented  epithelium 31 

26.  Glandular  epithelium 32 

27.  Rod-epithelium  ( Heidenhain  and  Schiefferdecker) 32 

28.  Isolated  neuro-epithelium 32 

29.  Endothelium 23 

30.  Endothelium  showing  stomata 34 

31.  Young  connective-tissue  cell 36 

32.  Embryonal  connective  tissue 36 

2^.  Subcutaneous  areolar  tissue 2>7 

34.  Special  connective-tissue  elements 2>7 

35.  Pigmented  connective-tissue  cells ^7 

36.  Pigment-cell 38 

37.  Plate-like  connective-tissue  cells 3S 

38.  Cell-spaces  of  dense  connective  tissue 38 

39.  Branched  connective-tissue  cells 38 

40.  White  fibrous  tissue  ....       39 

41.  Elastic  fibres  isolated  ( Schiefferdecker) 39 

42.  Young  connective-tissue  cells 40 

43.  Tendon  in  transverse  section 41 

xiii 


xiv  LIST   OF   ILLUSTRATIONS. 

FIG.  PAGE 

44.  Primary  bundles  of  tendon 41 

45.  Primary  tendon-bundles  in  section 42 

46.  Elastic  fibres  in  transverse  section 42 

47.  Elastic  fibres  forming  fenestrated  membrane 42 

48.  Subcutaneous  tissue  with  fat-cells 43 

49.  Hyaline  (costali  cartilage 44 

50.  Hyaline  cartilage  with  perichondrium 45 

51.  Elastic  cartilage 46 

52.  Fibro-cartilage 46 

53.  Transverse  section  of  dried  bone 47 

54.  Longitudinal  section  of  dried  bone  ...           48 

55.  Lacunae  and  canaliculi  of  dried  bone 48 

56.  Bone-cell  within  lacuna 49 

57.  Fragments  of  bone,  showing  Sharpey's  fibres 49 

58.  Cells  of  bone-marrow 50 

59.  Primary  embryonal  cartilage      51 

60.  Developing  bone — centre  of  ossification 52 

61.  Developing  bone — zone  of  calcification       53 

62.  Developing  bone — trabecular  of  endochondral  bone ...  54 

63.  Developing  bone — conversion  of  osteoblasts  into  bone-cells 54 

64.  Developing  bone — periosteal  and  endochondral  bone 55 

65.  Developing  bone — longitudinal  section  of  embryonal  phalanx  ......  55 

66.  Developing  bone — showing  Howship's  lacuna 56 

67.  Isolated  involuntary-muscle  cells 59 

68.  Involuntary-muscle  cells 59 

69.  Involuntary  muscle  in  transverse  section 60 

70.  Involuntary  muscle  in  longitudinal  section 60 

71.  Voluntary-muscle  fibres 61 

72.  Voluntary-muscle  fibres  in  section 62 

73.  Voluntary-muscle  fibres 62 

74.  Diagram  of  arrangement  of  contractile  substance 64 

75.  Muscle-fibres,  showing  Cohnheim's  fields 65 

76.  Voluntary  muscle  in  transverse  section 65 

77.  Branched  fibres  of  voluntary  muscle 65 

78.  Heart-muscle,  showing  branched  fibres      66 

79.  Heart-muscle  fibres  in  section 67 

80.  Injected  voluntary  muscle 67 

81.  Developing  voluntary  muscle 68 

82.  Nerve-cell  from  cerebral  cortex 70 

83.  Nerve-cell  isolated  from  spinal  cord 71 

84.  Nerve-cell  of  first  type 72 

85.  Nerve-cell  of  second  type 72 

86.  Basket-work  around  Purkinje's  cell 72 

87.  Nerve-cell  from  sympathetic  (Retzius) 73 

38.   Medullated  nerve-fibres ...  74 

89.  Ultimate  fibrillar  of  axis-cylinder 74 

90.  Medullated  nerve-fibres  treated  with  osmic  acid 75 

91.  Silvered  nerve-fibres 75 

92.  Nnn-medullated  nerve-fibres 76 

93.  Section  of  nerve-trunk      jj 

94.  Section  of  single  funiculus  of  nerve 78 

95.  Supporting  tissues  of  nerve-centres 79 

96.  Longitudinal  section  of  spinal  ganglion 80 

97.  Section  of  portion  of  spinal  ganglion 80 

98.  Ganglion  nerve-cell,  showing  spiral  fibre  {Schiefferdecker) 81 


LIST   OF   ILLUSTRATIONS.  xv 

FIG.  PAGE 

99.  Termination  of  sensory  nerve  fibres 83 

100.  Termination  of  sensory  nerve  fibres  within  the  epidermis 84 

101.  Special  nerve-endings  within  the  epidermis  (Ranvier) 84 

102.  Tactile  corpuscles— simple  and  compound 85 

103.  Tactile  corpuscle  of  Meissner  \Schiefferdecker) 85 

104.  Simple  spherical  end-bulb  (Krause) 86 

105.  Genital  corpuscle  (Krause) 86 

106.  Simple  cylindrical  end-bulbs  ( '  Schiefferdecker) 86 

107.  Corpuscle  of  Vater,  or  Pacinian  body  (Ranvier) 87 

108.  Herbst's  corpuscle 87 

109.  Nerves  of  involuntary  muscle 89 

no.  Nerves  of  voluntary  muscle 90 

in.  Motor  end-plate  of  voluntary  muscle 90 

112.  Golgi's  corpuscle,  or  tendon-spindle  1 Ciaccio) 91 

113.  Nerve-fibres  accompanying  a  small  artery 92 

114.  Nerves  ending  in  glands 93 

115.  Section  of  human  artery 94 

116.  Endothelium  of  artery  of  frog 94 

117.  Cell-spaces  of  intima  of  human  aorta 95 

118.  Fenestrated  membrane  of  intima  of  human  aorta 95 

119.  Muscle-cells  from  human  artery 96 

120.  Section  of  aorta  of  child 96 

121.  Small  arteries  and  capillary 97 

122.  Section  of  human  vein 98 

123.  Capillary  blood-vessels 99 

124.  Section  of  human  heart,  showing  endocardium 100 

125.  Section  of  human  heart,  including  valve 101 

126.  Section  of  human  heart,  including  pericardium 102 

127.  Developing  capillary  blood-vessels 103 

128.  Section  of  developing  heart 104 

129.  Human  colorless  blood-cells 105 

130.  Human  blood-cells  .    .   .    .    , 107 

131.  Red  blood-cells  of  man  and  of  amphiuma 109 

132.  Human  blood-cells,  showing  effects  of  reagents 109 

133.  Human  blood,  showing  blood-platelets,  fibrin,  etc no 

134.  Haemin  crystals  from  human  blood in 

135.  Lymph-spaces  within  fibrous  tissue  in  profile 115 

136.  Lymph-spaces  in  surface  view 116 

137.  .Lymph-capillary 116 

138.  Lymphatics  of  silvered  diaphragm 116 

139.  Perivascular  lymphatic  enclosing  an  artery 117 

140.  Section  of  human  thoracic  duct 117 

141.  Elements  of  adenoid  tissue 118 

142.  Diffuse  adenoid  tissue 119 

143.  Simple  lymph-follicle 119 

144.  Section  of  lymph-gland 120 

145.  Section  of  lymphatic  gland,  including  cortex         120 

146.  Section  of  lymphatic  gland,  including  medulla     121 

147.  Section  of  lymphatic  gland,  showing  details  of  structure 121 

148.  Section  of  spleen 123 

149.  Section  of  spleen,  showing  trabecular  and  reticulum 123 

150.  Section  of  large  trabecula  of  spleen 123 

151.  Section  of  human  spleen  cutting  a  Malpighian  corpuscle 124 

152.  Portion  of  channel  within  splenic  pulp 125 

153.  Diagram  of  relations  of  splenic  vessels  to  pulp-tissue 126 


xvi  LIST   OF   ILLUSTRATIONS. 

FIG.  PAGE 

154.  Section  of  human  thymus  body 127 

155.  Portion  of  periphery  of  follicle  of  thymus  body 127 

156.  Portion  of  same  follicle,  showing  Hassall's  corpuscles 127 

157.  Peritoneal  endothelium 129 

158.  Section  of  peritoneum 130 

159.  Section  of  synovial  membrane 131 

160.  Section  of  ten-day  rabbit  embryo 133 

161.  Diagram  of  typical  mucous  membrane 136 

162.  Cells  of  basement-membrane 137 

163.  Diagram  illustrating  form  of  glands 137 

164.  Tubular  glands     138 

165.  Section  of  racemose  gland 139 

166.  Section  of  human  parotid  gland     139 

167.  Serous  acini  of  parotid  gland 140 

168.  Mucous  acini  of  sublingual  gland 140 

169.  Section  of  lingual  glands 141 

170.  Serous  and  mucous  acini  of  glands 141 

171.  Developing  salivary  gland 142 

172.  Section  of  human  oral  mucous  membrane 144 

173.  Longitudinal  section  of  molar  tooth 146 

174.  Section  of  dried  tooth,  including  enamel  and  dentine 146 

175.  Interglobular  spaces  of  dentine 147 

176.  Section  of  enamel 147 

177.  Section  of  dried  tooth,  including  cementum  and  dentine 148 

178.  Section  of  young  tooth  and  pulp 149 

179.  Section  of  jaw  of  rabbit  embryo  with  early  dental  ridge 149 

180.  Model  of  embryonal  jaw  (Rose) 150 

181.  Section  of  jaw  of  rabbit  embryo — dental  ridge 150 

182.  Section  of  jaw  of  rabbit  embryo— enamel  organ 150 

183.  Section  of  jaw  of  cat  embryo — dental  papilla 151 

184.  Section  of  jaw  of  cat  embryo  with  four  developing  teeth 151 

185.  Section  of  enamel  organ  of  cat  embryo 152 

186.  Section  of  developing  tooth  of  cat  embryo 153 

187.  Section  of  human  tongue,  showing  conical  papillae 154 

188.  Section  of  human  tongue  with  fungiform  papilla 154 

189.  Section  of  circumvallate  papilla  from  child's  tongue 155 

190.  Section  of  taste-bud  from  circumvallate  papilla 155 

191.  Salivary  corpuscles  from  human  saliva 156 

192.  Section  of  tonsil  of  dog 157 

193.  Section  of  tonsil  of  child 157 

194.  Section  of  tonsil  of  child,  showing  structural  details 158 

195.  Section  of  human  oesophagus 161 

196.  Section  of  human  stomach 163 

197.  Peptic  gland  from  stomach  of  dog      163 

198.  Transverse  sections  of  gastric  glands  of  dog 164 

199.  Portion  of  peptic  gland  of  dog 164 

200.  Pyloric  glands  from  human  stomach 165 

201.  Section  of  pyloric  region  of  human  stomach 165 

202.  Section  through  pylorus  of  child's  stomach 165 

203.  Section  of  injected  stomach  of  cat     166 

204.  Nervous  plexuses  of  human  stomach  {Slohr) 167 

205.  Longitudinal  section  of  human  small  intestine 168 

206.  Tubular  glands  of  large  intestine  of  dog 169 

207.  Transverse  section  of  follicles  of  large  intestine 169 

208.  Longitudinal  section  of  villus  of  dog's  intestine 170 


LIST   OF   ILLUSTRATIONS.  xv[{ 

FIG.  PAGE 

209.  Transverse  section  of  villus  of  dog's  intestine 170 

210.  Longitudinal  section  of  large  intestine  of  child 171 

211.  Section  of  duodenum  of  cat 171 

212.  Section  of  human  large  intestine 172 

213.  Peyer's  patch  from  small  intestine  of  cat 172 

214.  Section  of  small  intestine  of  child 173 

215.  Section  of  injected  small  intestine  of  cat 174 

216.  Section  of  liver  of  hog 176 

217.  Section  of  human  liver 177 

218.  Diagram  of  structure  of  liver 177 

219.  Section  of  injected  human  liver 178 

220.  Hepatic  cells  from  human  liver 178 

221.  Section  of  uninjected  human  liver 178 

222.  Section  of  centre  of  lobule  of  human  liver 179 

223.  Section  of  liver  of  frog 179 

224.  Section  of  rabbit's  liver,  showing  bile-capillaries 180 

225.  Section  of  dog's  liver,  showing  interlobular  vessels 180 

226.  Transverse  section  of  large  bile-duct 181 

227.  Section  of  human  parotid  gland 183 

228.  Acini  of  human  parotid  gland 183 

229.  Section  of  human  sublingual  gland 184 

230.  Section  of  human  pancreas 185 

231.  Human  pancreas,  showing  area  of  immature  cells 186 

232.  Section  of  developing  gut  of  rabbit  embryo 187 

233.  Sagittal  section  of  nine-day  rabbit  embryo 188 

234.  Longitudinal  section  of  human  kidney  (Henle) 191 

235.  Section  of  human  kidney,  showing  general  arrangement 192 

236.  Section  of  partially-injected  human  kidney 193 

237.  Diagram  of  the  kidney 194 

238.  Uriniferous  tubules  of  human  kidney 196 

239.  Section  of  kidney  of  amphiuma 197 

240.  Constituents  of  medulla  of  human  kidney 197 

241.  Section  of  medulla  of  human  kidney 198 

242.  Section  across  papilla  of  human  kidney 199 

243.  Section  of  injected  kidney  of  dog 200 

244.  Transverse  section  of  human  ureter 201 

245.  Section  of  human  bladder 202 

246.  Developing  kidney  of  rabbit  embryo 204 

247.  Developing  kidney  of  rabbit  embryo 205 

248.  Developing  kidney  of  cow  embryo 205 

249.  Diagram  illustrating  structure  of  testicle 207 

250.  Section  of  human  testicle 208 

251.  Section  of  human  seminiferous  tubule 209 

252.  Spermatogenesis  in  testicle  of  dog 209 

253.  Spermatogenesis  in  testicle  of  musk-rat 210 

254.  Spermatogenesis  in  testicle  of  musk-rat 210 

255-  Spermatogenesis  in  testicle  of  musk-rat 211 

256.  Human  testicle,  showing  interstitial  cells 211 

257.  Section  of  tubule  of  human  epididymis 212 

258.  Section  through  epididymis  of  child 213 

259.  Human  spermatozoa 215 

260.  Human  spermatozoa,  highly  magnified 215 

261.  Section  of  penis  of  child 217 

262.  Erectile  tissue  of  human  penis 217 

263.  Section  of  human  prostate  gland 220 


xviii  LIST   OF   ILLUSTRATIONS. 

FIG.  PAGE 

264.  Human  prostate  gland,  showing  muscle 221 

265.  Section  of  ovary  of  cat 224 

266.  Human  ovary  with  Graafian  follicle 225 

267.  Section  of  cortex  of  cat's  ovary 226 

268.  Ovum  from  ovary  of  cat 227 

269.  Section  of  medulla  of  human  ovary 228 

270.  Section  of  corpus  luteum  of  rabbit 229 

271.  Portion  of  tubules  of  parovarium 230 

272.  Transverse  section  of  human  oviduct 231 

273.  Section  of  human  uterus 232 

274.  Section  of  uterine  cervix  of  child 233 

275.  Active  human  mammary  gland 238 

276.  Acini  of  active  human  mammary  gland 239 

277.  Atrophic  human  mammary  gland 240 

278.  Human  milk  and  colostrum-corpuscles 242 

279.  Wolffian  bodies  and  sexual  glands  of  rabbit  embryo 242 

280.  Indifferent  sexual  gland  of  rabbit  embryo 243 

281.  Section  of  developing  ovary  of  kitten 244 

282.  Diagram  illustrating  development  of  sexual  organs 245 

283.  Longitudinal  section  of  larynx  of  child 247 

284.  Longitudinal  section  of  epiglottis  of  child 248 

285.  Section  of  trachea  and  cesophagus  of  child 249 

286.  Section  of  human  bronchus 251 

287.  Diagram  of  air-passages  of  lung 251 

288.  Section  of  human  lung 252 

289.  Section  of  silvered  lung  of  kitten 253 

290.  Section  of  injected  and  inflated  lung  of  cat 254 

291.  Section  of  human  pleura 256 

292.  Section  of  thyroid  body  of  child      257 

293.  Acini  of  human  thyroid  body 258 

294.  Developing  pulmonary  tube  of  rabbit  embryo 259 

295.  Developing  lungs  of  rabbit  embryo 259 

296.  Developing  thyroid  body  of  rabbit  embryo 260 

297.  Developing  lateral  thyroid  area  of  rabbit  embryo 260 

298.  Section  of  human  skin 261 

299.  Epidermis  of  human  skin 262 

300.  Section  of  epidermis  of  human  skin 262 

301.  Section  of  negro's  skin 264 

302.  Section  of  child's  finger,  including  nail ....  266 

303.  Section  of  human  scalp ...  267 

304.  Human  hair 268 

305.  Hair-follicle  from  human  scalp 269 

306.  Transverse  sections  of  hair-follicles 270 

307.  Section  of  hair-follicle  near  its  mouth 270 

308.  Section  of  hair-follicle,  highly  magnified 271 

309!  Section  of  sebaceous  gland  from  human  scalp 273 

310.  Section  of  human  sweat-gland 275 

311.  Developing  skin  from  human  foetus 277 

312.  Developing  skin  from  foetal  kitten 279 

313.  Developing  hair  from  fcetal  kitten 279 

314.  Section  of  skin  of  fcetal  kitten 279 

315.  Degenerating  hair-follicle  from  human  scalp 280 

316.  Section  of  skin  of  human  foetus 281 

317.  Brain-membranes  of  child 282 

318.  Section  of  human  spinal  cord  from  cervical  region 285 


LIST   OF   ILLUSTRATIONS.  xix 

FIG.  PAGE 

319.  Diagram  of  fibre-tracts  of  spinal  cord 286 

320.  White  matter  of  spinal  cord     287 

321.  Section  of  human  spinal  cord  from  thoracic  region 288 

322.  Section  of  human  spinal  cord  from  lumbar  region 289 

323.  Central  canal  and  commissures  of  spinal  cord  of  calf 289 

324.  Anterior  horn  of  gray  matter  of  spinal  cord  of  man        290 

325.  Anterior  horn  of  gray  matter  of  spinal  cord  of  calf 291 

326.  Diagram  of  cells  and  fibres  of  spinal  cord  {Lenhossek) 292 

327.  Neuroglia  cells  of  embryonal  spinal  cord  ^Lenhossek) 294 

328.  Section  of  injected  human  spinal  cord 295 

329.  Diagram  of  decussations  of  medulla  (  Testut) 296 

330.  Diagram  of  decussating  tracts  of  medulla  (  Testut-Duval) 297 

331.  Diagram  of  medulla  through  olivary  bodies  {Testut-Duval) 297 

332.  Diagram  of  sensory  decussation  of  medulla  {Testut-Duval) 298 

2,2,2,.  Diagram  of  medulla  through  olivary  bodies  {Te stut-Duval) 298 

334.  Section  of  medulla  of  child 299 

335.  Section  through  human  pons  {Testut- Stilling) 302 

336.  Section  through  human  cerebral  peduncle  {Krause) 303. 

237-  Section  of  human  cerebellum 305 

338.  Diagram  of  nerve-cells  of  cerebellum 306 

339.  Section  of  human  cerebellum 307 

340.  Section  of  cerebellar  cortex  of  dog  {Retzius) 309 

341.  Section  of  human  cerebral  cortex 312 

342.  Section  of  silvered  human  cerebral  cortex 313 

343.  Nerve-fibres  of  human  cerebral  cortex 314 

344.  Section  across  cornu  Ammonis  {Henle) 316 

345.  Diagram  of  constituents  of  cornu  Ammonis  {K.  Schaffer) 317 

346.  Section  across  optic  thalamus  {Schwalbe-Meynert) 320 

347.  Section  across  corpora  quadrigemina  {Quain-Meynert)     ...    T   ...    .  322 

348.  Section  of  human  olfactory  bulb  {Henle) 324 

349.  Section  of  olfactory  bulb  of  rabbit  {Retzius) 325 

350.  Diagram  of  cerebral  association  fibres  {Schaefer-Mey7ierl) 326 

351.  Section  of  human  pituitary  body 328 

352.  Section  of  pineal  sense-organ  of  lizard  embryo 329 

353.  Section  of  human  pineal  body 330 

354.  Corpora  amylacea  from  human  brain 330 

355.  Section  of  human  suprarenal  body 331 

356.  Section  of  rabbit  embryo,  showing  open  neural  tube 332 

357.  Section  of  rabbit  embryo,  showing  closed  neural  tube 333 

358.  Primary  wall  of  neural  tube  {His) 333 

359.  Germ-cells  and  neuroblasts  {His) 333 

360.  Germ-cells  and  spongioblasts  {His) 334 

361.  Spongioblasts  from  neural  tube  {His) 334 

362.  Section  of  human  cornea 337 

363.  Fibrous  tissue  of  cornea  of  ox 33$ 

364.  Corneal  corpuscles  of  calf 33S 

365.  Corneal  spaces  of  calf 339 

366.  Corneal  spaces  of  calf 339 

367.  Plexus  of  corneal  nerves 340 

368.  Section  of  walls  of  human  eyeball 341 

369.  Section  of  human  choroid 342 

370.  Human  choroid  from  surface 343 

371.  Section  of  human  ciliary  processes 344 

372.  Section  through  ciliary  region  of  human  eye 345 

373.  Section  of  iris  and  lens  of  human  eye 347 


XX 


LIST   OF   ILLUSTRATIONS. 


FIG.  PAGE 

374.  Injected  iris  from  eye  of  dog 348 

375.  Irido-corneal  angle  of  human  eye 350 

376.  Diagram  of  retinal  elements  (Kallius  after  Ramon  y  Cajal) 352 

377.  Section  of  human  retina 353 

378.  Human  retina  at  macula  lutea  {Max  Schultze) 357 

379.  Human  retina  at  ora  serrata 358 

380.  Transverse  section  of  human  optic  nerve 359 

381.  Section  of  part  of  human  optic  nerve 359 

382.  Longitudinal  section  of  human  optic  entrance 360 

383.  Portions  of  human  crystalline  lens     362 

384.  Fibres  of  human  crystalline  lens 362 

385.  Section  through  anterior  segment  of  human  eye 363 

386.  Section  of  human  eyelid 368 

387.  Section  of  human  lachrymal  gland 371 

388.  Primary  optic  vesicle  of  rabbit  embryo t>72> 

389.  Developing  lens  and  optic  cup  of  rabbit  embryo 373 

390.  Choroidal  fissure  in  developing  eye  of  rabbit  embryo 373 

391.  Developing  eye  of  rabbit  embryo 375 

392.  Section  through  developing  eye  of  rabbit  embryo 376 

393.  Section  of  human  external  auditory  canal  (Riidinger) 378 

394.  Human  tympanic  membrane  and  malleolus  [Riidinger) 379 

395.  Section  of  human  Eustachian  tube  (  Testut) 382 

396.  Section  of  membranous  labyrinth  of  cat 384 

397.  Section  of  utricle  of  rabbit,  showing  otoliths 385 

398.  Section  of  semicircular  canal  of  cat 386 

399.  Membranous  semicircular  canal  of  cat 387 

400.  Longitudinal  section  of  cochlea  of  guinea-pig 388 

401.  Section  of  cochlea  of  cat 390 

402.  Section  of  Corti's  organ  of  guinea-pig 392 

403.  Diagrammatic  view  of  Corti's  organ  (  Testut) 393 

404.  Section  through  auditory  pit  of  rabbit  embryo 398 

405.  Section  through  otic  vesicle  of  rabbit  embryo 39S 

406.  Section  through  developing  ear  of  rabbit  embryo 399 

407.  Section  through  developing  cochlea  of  rabbit  embryo 400 

408.  Section  of  human  respiratory  nasal  mucous  membrane 402 

409.  Section  of  olfactory  mucous  membrane  of  child 403 


NORMAL  HISTOLOGY. 


CHAPTER    I. 

THE    CELL   AND    THE   TISSUES. 

Histology,  literally,  the  science  of  tissues,  represents  that  part 
of  general  morphology  which  treats  of  the  structural  elements  of 
organisms,  by  the  various  arrangement  of  which  the  textures  and 
organs  of  the  body  are  formed.  The  term  is,  evidently,  equally 
applicable  to  the  structural  components  of  plants  as  well  as  to  those 
of  animals;  "histology,"  however,  is  usually  accepted  as  relating 
especially  to  animal  tissues,  ' '  vegetal  histology' '  expressing  the 
extension  of  the  study  to  the  tissues  of  plants. 

At  first  sight  apparently  complex  and  numerous,  the  structures 
composing  the  animal  economy  are  really  made  up  of  but  few 
elementary  tissues  ;  these  latter  may  be  divided  into  four  funda- 
mental groups: 

Epithelial  tissues; 

Connective  tissues; 

Muscular  tissues; 

Nervous  tissues. 

Each  of  these  tissues  may  be  further  resolved  into  the  compo- 
nent morphological  constituents,  the  cells  and  the  intercellular 
substances.  All  animal  cells  are  the  descendants  of  the  embryonal 
elements  derived  from  the  division  of  the  primary  parent  cell — the 
ovum;  the  intercellular  substances,  on  the  other  hand,  are  formed 
through  the  more  or  less  direct  agency  of  the  cells.  The  animal 
cell  may  exist  in  either  the  embryonal,  matured,  or  metamorphosed 
condition. 

The  embryonal  cell,  as  represented  by  the  early  generations  of 
the  direct  offspring  of  the  ovum,  or  by  the  lymphoid  or  colorless 
blood-cells  of  the  adult,  is  a  pIG  lm 

small    irregularly   round    or 
oval  mass  of  finely  granular  &0m, *.. 
gelatinous     substance  —  the   ^^=-/       6^33^ 

. 1  11  Colorless  blood-cell  exhibiting  amoeboid  movement. 

protoplasm  or  cell-contents 

— in  some   part  of  which  a  smaller  and  often  indistinct  spherical 

body — the  nucleus — lies  embedded.     In  the  embryonal  condition, 


12 


NORMAL   HISTOLOGY. 


Fig.  2. 


when  the  cell  is  without  a  limiting  membrane  and  composed  al- 
most entirely  of  active  living  substance,  the  outlines  are  frequently- 
changing,  these  variations  in  shape  being  known  as  amoeboid 
movements,  from  their  similarity  to  the  changes  observed  in  the 
outline  of  an  active  amoeba,  one  of  the  simplest  forms  of  animal  life. 
As  the  embryonal  cell  advances  in  its  life-history,  the  surrounding 
conditions  to  which  it  is  subjected  induce,  with  few  exceptions, 
further  specialization.  Among  the  earliest  of  such  effects  is  the 
condensation  of  the  peripheral  zone  of  the  cell,  whereby  the  reten- 
tion of  a  definite  form  is  greatly  favored:  such  peripheral  condensa- 
tion may  progress  to  the  production  of  a  distinct  limiting  membrane 
— the  cell-wall.  This  structure  is  very  frequently  wanting;  when 
present,  however,  it  is  usually  so  thin  that  its  optical  expression  is  a 
single  delicate  line.  The  cell-wall  is  to  be  regarded  as  a  product  of 
the  specialization  of  a  portion  of  the  protoplasm,  rather  than  as  an 
essential  part  of  the  cell. 

The  adult  cell  consists  of  the  protoplasm,  or  cell-contents,  possibly 

limited  by  a  cell-wall,  en- 
closing a  nucleus,  which 
latter,  in  turn,  often  con- 
tains one  or  more  minute 
spherical  bodies,  the  nu- 
cleoli. The  more  or  less 
definite  and  characteristic 
forms  which  the  elements 
of  the  various  tissues 
possess  on  reaching  their 
full  development,  depend 
largely  upon  the  changes 
effected  by  growth  and  dif- 
ferentiation in  the  proto- 
plasm during  the  younger 
condition  of  the  cells. 

The  protoplasm  of 
which  the  greater  part  of 
cells  is  composed,  using 
the  term  in  its  broadest 
application  and  as  sy- 
nonymous with  cell-contents,  usually  appears  as  a  finely  granular 
semi-fluid  or  gelatinous  substance,  in  which  darker  and  coarser 
granules  or  other  particles  of  extraneous  matters  are  often  embedded. 
The  structure  of  protoplasm  is  now  recognized  as  farlmore  com- 
plicated than  was  formerly  supposed,  comprising  a  highly  elastic 
and    extensible   portion — the   spongioplasm — and   an    interstitial, 


Typical  cell, — ovum  of  cat :  a,  protoplasm  ;  b,  nucleus  ;  c, 
nuclear  membrane  ;  d,  nucleolus ;  e,  true  cell-wall,  closely 
applied  to  the  surrounding  secondary  envelope,  the  zona 
pellucida. 


THE    CELL    AND    THE    TISSUES. 


13 


seemingly  less  active  substance — the  hyaloplasm.  The  active 
contractility  which  has  been  generally  credited  to  the  spongioplasm 
has  been  recently  questioned  (Schaefer),  since  the  characteristic 
amoeboid  movements  of  living  cells  are  by  some  attributed  to  the 
changes  taking  place  within  the  hyaloplasm. 

The  arrangement  of  these  constituents  of  the  protoplasm  is  vari- 
able. When  they  exist  closely  and  uniformly  intermingled,  the 
customary  finely  granular  appearance  of  the  cell-contents  is  produced; 
not  infrequently,  however,  the  spongioplasm  is  disposed  as  a  more 
or  less  well-defined  reticulum.  In  living  cells  this  reticulation  is 
transient,  and,  to  a  certain  degree,  acci- 
dental,  since  it  often  depends  upon  an  j.„^ 

unequal  distribution  of  the  hyaloplasm 
induced  by  the  presence  of  vacuoles  or 
of  particles  of  foreign  substance,  as  se- 
cretion within  glandular  epithelium. 

Chemically,  protoplasm  consists  of 
various  albuminous  substances  in  com- 
bination with  a  special  nitrogenous  pro- 
teid,  plastin,  together  with  water  and 

SaltS.        It    is    probable    that    in     the    albu-  Structure  of  the  cell  :   a,  spongio- 

minous  substances  alone  the  property  of    Jj^^"^"^^^^ 

Contractility    resides;    the    plastin,    On    the       wall ;  c,  chromatin  filaments,  between 
.1  ijrr  i.  *^„„     j-„       which  lies  nuclear  matrix ;   a',  nuclear 

other   hand,    offers    great   resistance   to     membrane;  ,>nucleo!us. 
those  reagents,  as  acids,   gastric  juices, 

or  trypsin,  which  dissolve  the  albuminates.  The  amount  of  plastin 
present  within  the  fibrils  forming  the  intercellular  reticulum  is  not 
constant,  but  subject  to  considerable  variation.  In  addition  to  the 
hyaloplasm,  the  meshes  of  the  spongioplasm  frequently  contain 
particles  of  foreign  substances  ;  the  latter  may  be  fatty  matters, 
pigment  granules,  particles  of  secretion  elaborated  within  the  cell 
itself,  or  extraneous  material. 

The  nucleus  is  limited  by  a  distinct  wall,  the  nuclear  membrane, 
and  is  traversed  by  a  variably  elaborate  framework  of  nuclear 
fibrils,  between  which  lies  an  interfibrillar,  probably  semi-fluid,  sub- 
stance, the  nuclear  matrix.  The  fibrils  are  the  more  active  con- 
stituents of  the  nucleus,  since  they  take  a  conspicuous  part  in  the 
changes  attending  the  division  of  the  cell.  In  recognition  of  the 
marked  affinity  for  certain  dyes  possessed  by  these  threads,  the 
substance  composing  the  fibrils  is  often  termed  chromatin,  while 
the  but  slightly  staining  nuclear  matrix  is  designated  achromatin. 

Suspended  within  the  nuclear  net-work,  lying  often  in  close  rela- 
tion with  the  fibrils,  one  or  more  minute  spherical  bodies  may  be 
seen;  these  are  the  nucleoli,  regarding  whose  true 'significance,  at 


H 


NORMAL    HISTOLOGY. 


present,  little  is  definitely  established.  The  nucleolus  is  highly 
refracting,  and,  when  subjected  to  appropriate  stains,  takes  on  a 
color  differing  from  both  nucleus  and  protoplasm,  suggesting,  at 
least,  a  distinct  chemical  condition.  This  body  lies  closely  approxi- 
mated to,  but  separated  from,  the  nuclear  fibrils,  being  an  indepen- 
dent member  of  the  cell;  this  fact  is  especially  evident  in  such  ele- 
ments as  ganglionic  nerve-cells,  or  ova,  where  the  nucleolus  appears 
with  exceptional  distinctness.  Its  disappearance  during  the  division 
of  the  nucleus,  and  its  subsequent  reappearance  within  the  newly- 
formed  nuclei,  lend  weight  to  the  supposition  that  the  nucleolus  plays 
but  a  subordinate  rdle  in  the  life-history  of  the  cell;  its  true  value, 
however,  has  yet  to  be  determined. 

In  addition  to  the  parts  of  the  cell  generally  recognized,  recent 
investigators  have  described  the  occasional  presence  of  an  irregularly 
spherical  body,  lying  within  the  protoplasm  in  the  vicinity  of  the 


Fig.  4. 


Fig.  5. 


A 


B 


A,  cell  from  pancreas  of  salamander :  n, 
nucleus ;  /,  paranucleus.  B,  sexual  cell  of 
leech  :  «,  nucleus  ;  /,  paranucleus  ;  c,  centro- 
some.     (After  Plainer.) 


Segmenting  ova  of  ascaris 
megalocephala  :  n,  nucleus  ; 
a,  centrosome,  surrounded 
by  attraction-sphere  ;  p,  po- 
lar body.     (After  Boveri.) 


nucleus,  to  which  the  name  accessory  nucleus,  or  paranucleus 
(Nebe?iker?i  of  the  Germans),  has  been  applied.  According  to  Plat- 
ner,  the  paranucleus  is  an  extrusion  of  the  nucleus,  and  is  subject  to 
great  variation  in  size  and  appearance;  the  nature  and  function  of 
this  body  are  at  present  still  obscure,  and  need  further  investigation. 
Likewise,  the  presence  of  a  very  small,  round,  highly-refracting 
body — the  centrosome,  or  pole-corpuscle — has  been  established  in 
sexual  cells,  and  also  in  many  other  elements.  The  centrosome  is 
itself  surrounded  by  an  area  named  the  attraction- sphere.  While 
these  bodies  have  been  shown  to  exist  during  the  condition  of  rest, 
it  is  especially  in  connection  with  the  changes  incident  to  the  division 
of  the  nucleus  that  their  most  conspicuous  features  have  been  ob- 
served; much,  however,  remains  to  be  determined  regarding  these 
constituents  of  the  cell. 


THE    CELL   AND    THE   TISSUES. 


15 


THE   VITAL   MANIFESTATIONS    OF   THE    CELL. 

The  characteristics  which  distinguish  the  structural  units  of  living 
organisms  from  those  of  the  inorganic  world,  may  be  conveniently 
grouped  as — Vegetative,  Metabolism,  Growth,  Reproduction;  Ani- 
mal, Irritability,  Motion. 

Metabolism  is  that  process  by  which  the  cell  selects  and  assimi- 
lates, from  the  surrounding  food-materials,  those  substances  adapted 
to  the  particular  needs  for  its  nutrition  and  function,  so  changing 
and  incorporating  into  its  own  substance  the  materials  so  acquired 
that  they  become  an  integral  part  of  the  cell.  By  a  still  further 
exercise  of  this  process  the  assimilated  materials  are  converted  into 
new  substances,  which  may  be  retained  within  the  cell,  or,  as  is 
frequently  the  case,  given  up  as  the  various  secretions  of  the  body. 

Growth,  the  natural  sequence  of  assimilation,  may  affect  the  cell 
equally  in  all  parts,  thereby  producing  a  uniformly  enlarged  ele- 
ment; such  normal  or  typical  increase  is,  as  a  rule,  hindered  by  the 
impression  of  neighboring  elements,  such  limitations  resulting  in 
many  local  alterations  of  form,  as  conspicuously  seen  in  epithelial 
tissues.  It  is,  however,  the  principle  of  unequal  growth  that  exerts 
the  greatest  influence  in  producing  specializations  of  form,  as  exam- 
ples of  which  the  cells  of  muscle,  the  crystalline  lens,  or  connective 
tissue  are  familiar. 

Reproduction,  the  culminating  phenomenon  of  the  life-history  of 
the  cell,  occurs  by  two  modes  : 

a.  By  direct  division — without  karyokinesis. 

b.  By  indirect  division — with  karyokinesis. 

Direct  division,  by  which  a  cell  rich  in  protoplasm,  as  the  white 
blood-corpuscle,  constricts,  cuts  off,  and  sets  free  a  portion  of  itself, 
while    undoubtedly 

taking  place  in  the  FlG-  6. 

multiplication  of  the 
simplest  organisms, 

Or    OI     the    least    dll-  Direct  cell-division  of  colorless  blood-corpuscle. 

ferentiated  elements 

of  higher  types,  is  no  longer  regarded,  as  formerly,  as  the  most 
important  and  usual  mode  of  cell  reproduction  ;  the  observations 
of  the  last  decade  have  shown  that  its  occurrence  must  be  accepted 
rather  as  exceptional  than  as  customary. 

Indirect  division,  preceded  by  the  complicated  cycle  of  nuclear 
changes  collectively  termed  karyokinesis,  is  now  recognized  as 
being  the  usual  mode  of  the  reproduction  of  cells  of  all  kinds,  in 
pathological  as  well  as  in  normal  conditions.  The  recognition  and 
elucidation  of  these  important  phenomena  have  been  largely  due  to 


i6 


NORMAL   HISTOLOGY. 


Fig.  7. 
P 


the  brilliant  investigations  of  Flemming,  Strasburger,  v.  Beneden, 
Schleicher,  Rabl,  and  others,  who,  by  the  employment  of  improved 
optical  appliances  and  methods  of  investigation,  have  added  much 
to  the  accurate  knowledge  of  the  life-history  of  the  cell. 

When  the  cell  undergoes  a  complete  and  typical  mitotic  division, 
the  following  changes  occur: 

(1)  The  nucleus  becomes  larger,  and,  at  the  same  time,  the  chro- 
matin greatly  increases,  the  fibrils  becoming  con- 
torted to  form  a  dense  convolution,  whose  twisted 
threads  run  generally  transverse  to  the  long  axis  , 
of  the  nucleus  and  parallel  to  the  plane  of  the  ' 
future  cleavage;  these  fibrils  constitute  the 
(2)  Close  skein,  or  spirem.    The  chromatin 
fibrils  further  thicken,  be- 
Fig.  8.  coming    less    convoluted, 

Pv  and    forming    irregularly- 

arranged  loops,  known  as 
the 

(3)  Loose  skein.  The 
question  whether  these 
skeins  are  composed  of  the 
contortions  of  one  long 
fibre,  or  whether  they  con- 
tain several  shorter  ones, 
has,  as  yet,  not  been  defi- 
nitely determined ;  observa- 
tions made  on  the  cells  of 
lower  forms,  however,  ren- 
der it  not  improbable  that 
a  single  thread  constitutes 
the  entire  convolution. 
The  fibrils  of  the  loose 
skein  now  separate  at  their 
peripheral  turns,  so  that  a 
number — about  twenty- 
four  (Flemming,  Rabl) — of  distinct  loops  are  formed;  the  closed 
ends  of  these  are  directed  towards  a  common  centre,  around  which, 
but  removed  some  little  distance,  they  become  arranged.  The 
enclosed  clear  space  is  the  polar  field.  During  the  formation  of 
the  skeins  the  nuclear  membrane  disappears,  its  former  position 
being  marked  for  some  time  longer  by  a  clear  zone  or  halo  surround- 
ing the  nucleus  and  defining  the  boundary  of  the  latter  from  the 
cell-contents.  Coincidently  with  the  formation  of  the  loose  skein,  a 
very  important  phenomenon  takes  place.      Within  the  achromatin 


Close  skein, — diagram  of 
nuclear  fibrils:  A,  seen 
from  the  side  ;  B,  from  the 
polar  field,  P  ;  C,  from 
anti-pole,  GP.  ( After  Rabl- 
Schiefferdecker .) 


Loose  skein  :  nuclear  spin- 
dle has  appeared  in  polar 
field,  P.  (After  Rabl-Schief- 
ferdecker.) 


THE   CELL   AND    THE   TISSUES. 


17 


delicate  striae  make  their  appearance,  so  disposed  that  together  they 
present  a  double  cone,  whose  apices  are  directed  towards  the  poles 
of  the  future  new  nuclei,  and  whose  bases  are  placed  centrally  and 
occupy  the  polar  field;  these  achromatin  figures  constitute  the  nu- 
clear spindle.  The  chromatin  fibrils  grow  thicker  and,  at  the 
same  time,  shorter,  and  arrange  themselves  so  that  the  closed  ends 
of  the  loops  encircle  the  polar  field,  giving  rise,  when  seen  from  its 
surface,  to  the  wreath;  seen  from  the  side,  however,  the  loops  or  V's 
appear  as  radiating  fibrils,  and  constitute  the 

(4)  Mother-star,  or  aster:  the  apparent  differences,  therefore, 
between  the  wreath  and  the  aster  depend  upon  the  point  of  view, 
and  not  upon  variations  in  the  arrangement  of  the  fibres.  Another 
very  important  change  is  now  observed. 

(5)  Each  of  the  loops  undergoes  longitudinal  cleavage,  split- 
ting up  into  double  the  number  of  segments:  these  are -now  entirely 
rearranged,  the  first  step  being 

(6)  A  rapid  separation  into  two  groups,  passing  towards  the  poles 
of  the  future  new  nuclei,  as  indicated  by  the  foci  of  the  nuclear 
spindle.     Around  these  points  as  centres,  a  deli-  fig.  9. 
cate  radial   marking — the   polar  striation — ap-  p 
pears.       The   halves   of   the    longitudinally-cleft 
fibrils  are  so  disposed  that  one  of  each  pair  of    fa"" 
sister-segments  passes  along  the  guiding  lines  of  If 
the  achromatin  spindle  to  each  of  the  groups,  thus  li 
insuring  an  accurate  and  equal  division  of  the 
original  chromatin  between  the  new  nuclei.     The 
chromatic  segments,  becoming  further  aggregated 
about  the  equator  of  the  nuclear  spindle  in  their 
migration,  form  a  compressed  mass,  known  as  the 

(7)  Equatorial  plate.*  As  the  newly-grouped 
fibrils  pass  outward  towards  their  respective  poles, 
the  free  ends  of  the  receding  segments  become 
united  by  delicate  threads  of  achromatin — the 
connecting  filaments — which  stretch  between 
the  corresponding  limbs  of  the  separating  seg- 
ments. With  the  completion  of  migration  the 
cardinal  features  of  the  division  of  the  nucleus 
have  been  established,  since  the  subsequent 
stages  are  but  repetitions,  in  inverse  order,  of  the 
changes  already  instituted.  Following  the  stage 
of  the  equatorial  plate,  the  fibrils  group  themselves  about  the  poles 
of  the  spindle  and  form 

*  The  term  "  equatorial  plate"  has  been  employed  by  some  authors  to  indicate 
the  later  phases  of  the  aster  stage. 


GP 

Rearrangement  and 
cleavage  of  V-segments  : 
A ,  from  the  side ;  B . 
from  the  polar  field,  P ; 
GP,  anti  -  pole.  (After 
Rabl-Schieffer  decker.) 


xg  NORMAL   HISTOLOGY. 

(8)  The  daughter- stars,  or  diaster,  each  of  these  corresponding 
to  a  new  nucleus.  About  this  time  the  cell-protoplasm,  which  until 
now  has  been  almost  passive,  begins  to  exhibit  a  constriction  of  its 
body,  which  impression  now  steadily  progresses  until  the  protoplasm 
of  the  cell  completely  separates  into  the  portions  destined  to  become 
the  bodies  of  the  cells,  enveloping  the  new  nuclei.  The  karyokinetic 
cycle  is  completed  by  each 

(9)  Daughter-wreath  or  star  in  turn  assuming 

(10)  The  stage  of  the  daughter-skeins,  at  first  loose  and  afterwards 
close;  on  obtaining  nuclear  membranes  and  the  nucleoli  reappearing, 
the  new  nuclei  finally  pass  into  the  stage  of  rest. 


Fig.  10. 


B 


Diagram  illustrating  the  migration  and  redisposition  of  the  segments  of  chromatin,  guided  by  the  achro- 
matic lines  :  A ,  mother-star  ;  B  and  C,  stage  of  equatorial  plate  ;  D,  daughter-stars.    (After  Rabl.) 

In  recapitulation,  the  above  changes  may  be  tabulated  as  follows: 
Resting  Mother-Nucleus  :  the  inauguration  of  the  changes 
leading  to  division  are  marked  by  increase  of  chromatin, 
resulting  in  the  formation  of 

1.  The  Mother- Skein  {Spirent): 

a.  Close  skein, — 

Disappearance  of  nucleoli. 
Disappearance  of  nuclear  membrane. 

b.  Loose  skein, — 

Separation  of  skein  into  segments. 

Appearance  of  polar  field. 

Rearrangement  of  segments  around  polar  field  to  form 

2.  The  Mother-Wreath,  or  Aster  : 

Appearance  of  nuclear  spindle. 

Longitudinal  cleavage  of  chromatin  segments. 

3.  Migration  of  Segments  (Metakinesis): 

Segments  pass  towards  the  poles  of  the  new  nuclei. 
Equatorial  plate  produced  by  massing  of  migrating  seg- 
ments. 


THE   CELL   AND   THE   TISSUES. 

Separation  of  segments  into  polar  groups. 
Appearance  of  connecting  filaments. 

4.  Daughter-Wreaths,  or  Asters  : 

Beginning  division  of  cell-protoplasm. 

5.  Daughter- Skeins : 

a.  Loose  skein. 

b.  Close  skein. 

Completion  of  new  nuclei. 
Acquisition  of  nuclear  membranes. 
Reappearance  of  nucleoli. 
Completed  separation  of  cell-protoplasm. 
Resting  Daughter- Nuclei. 


19 


Fig.  11. 


G 


K 


Cells  from  the  epidermis  of  very  young  larva  of  newt :  A ,  resting  nucleus  ;  B,  close  skein  ;  C,  loose 
skein  ;  D  and  E ,  mother-stars,  seen  from  the  polar  field  and  appearing  as  the  wreath  stage  ;  J*,  mother- 
star  from  the  side  ;  G,  migration  of  segments  ;  H,  daughter-stars  ;  /and  J,  segments  grouped  about 
new  polar  fields  (in  J  the  protoplasm  exhibits  constriction) ;  K,  daughter-skeins, — division  of  nucleus 
complete  with  slight  constriction  of  cell-body ;  L,  completed  division  of  nucleus  and  protoplasm. 


As  closely  connected  with  the  division  of  the  ovum,  and  probably, 
also,  with  that  of  many  other  cells,  the  behavior  of  the  minute  extra- 
nuclear  bodies — the  centrosomata  (Boveri),  or  pole-corpuscles 
(v.  Beneden),  and  their  surrounding  attraction-spheres  —  has 
attracted   the  attention  of  recent  investigators.     The   centrosome 


20 


NORMAL   HISTOLOGY. 


during  the  resting  stage  is  single,  but  its  multiplication  early  takes 
place  in  the  dividing  nucleus  and  anticipates  the  establishment  of  the 
poles  of  the  new  nuclei;  the  apices  of  the  nuclear  spindles  coincide 
with  the  attraction-spheres,  which  are,  probably,  potent  factors  in 
determining  the  exact  position  of  the  spindles  and,  consequently,  the 
plane  of  division. 

Fission  of  the  nucleus  is  ordinarily  followed  by  cleavage  of  the 
protoplasm,  the  resulting  new  cells  being  entirely  distinct  elements. 

A  deviation  from  this  usual 
procedure  is,  however,  some- 
times encountered  where  the 
division  of  the  nucleus  has 
not  been  followed  by  cleavage 


Fig 


Fig 


Segmenting  ova  of  ascaris  megalocephala  :  A  ,  cell  Large   marrow-cell  :    the   nil- 

contains  nucleus,  two  centrosomes  (c),  surrounded  by  cleus    has   undergone   repeated 

attraction-spheres,  and  adherent  polar  body  (/S) ;   B,  division  without  cleavage  of  the 

beginning  polar  striation  around  the  centrosomes  and  protoplasm, 

attraction-spheres  ;  C,  cell  viewed  from  polar  field,  the 
striation  proceeding  from  the  centrosome  ;  D,  cell  seen 
from  the  side,  apices  of  nuclear  spindle  correspond  with 
centrosomes.     (After  Boveri.) 

of  the  cell-protoplasm,  the  latter  remaining  undivided  even  after  the 
repeated  division  of  the  nuclei.  Examples  of  such  "endogenous" 
formation  are  seen  in  the  multinucleated  giant  marrow-cells. 

These  complicated  phenomena  can  be  satisfactorily  observed  only 
in  suitable  preparations  and  with  adequate  optical  appliances;  the 
dividing-cells  of  the  surface  epithelium  of  very  young  larval  newts 
(ten  to  twenty  millimetres  long)  supply  admirable  views  of  all  stages 
of  karyokinesis.  In  order  to  obtain  permanent  preparations,  how- 
ever, these  transient  changes  must  be  "  fixed"  by  powerful  reagents, 
insuring  the  instantaneous  death  of  the  tissue  (see  Appendix); 
otherwise  the  cycle,  which  occupies  only  from  two  to  three  hours, 
and  often  even  less  time,  will  have  been  completed,  and  all  trace  of 
the  figures  lost.  The  command  of  at  least  five  hundred  diameters, 
with  unexceptionable  definition,  is  likewise  essential  for  the  careful 
study  of  these  changes.  While  most  favorably  seen  in  fixed  and 
stained  preparations,  the  karyokinetic  figures  may  be  observed  in 
living  cells,  thus  proving  that  they  in  no  wise  depend  upon  reagents 
for  their  existence. 


THE   CELL   AND    THE    TISSUES.  21 

The  foregoing  vital  manifestations,  being  chiefly  concerned  in  the 
mere  existence  and  perpetuation  of  the  cell,  are  appropriately  termed 
vegetative  ;  irritability  and  motion,  on  the  contrary,  are  the  ex- 
pressions of  a  higher  and  more  individual  existence,  and  hence  are 
called  animal.  It  is  to  be  remarked  that  the  term  "animal,"  as 
here  employed,  must  not  be  regarded  as  indicating  distinctions  be- 
tween plants  and  animals;  for  this  purpose  such  manifestations  are 
inadequate,  since  the  elements  of  certain  plants  (Mimoseae,  Dionaea) 
possess  irritability,  and  the  protoplasm  of  others  (Myxomycetes, 
Volvocineae)  exhibits  motion  in  a  marked  degree. 

Irritability  is  that  property  of  living  matter  by  virtue  of  which 
external  influences  are  responded  to  by  changes  within  the  cell;  these 
changes  may,  in  turn,  induce  secondary  phenomena.  Instances  of 
such  impressions  are  frequent  among  the  lower  forms,  where  surface 
elements,  or,  as  among  the  still  simpler  unicellular  protozoa,  the  pe- 
ripheral zone  of  the  protoplasm  common  to  the  entire  animal,  exhibit 
susceptibility  to  external  stimuli.  Among  the  higher  animals  irri- 
tability is  manifested  by  nerve-cells,  which,  through  their  processes, 
influence  other  tissues.  Concerning  the  exact  nature  of  the  intimate 
changes  taking  place  within  the  cell,  the  sum  of  which  we  call  nervous 
phenomena,  little  is  known;  it  is  probable,  however,  that  the  al- 
buminous constituents  of  the  protoplasm  are  the  particular  seat  of 
these  obscure  molecular  changes. 

Motion,  more  or  less  pronounced,  is  a  characteristic  of  all  ani- 
mal cells — and,  likewise,  of  very  many  vegetal  ones — during  some 
portion  of  their  existence.  The  development  and  specialization  of  the 
adult  cell  usually  result  in  limitation  of  the  activity  of  the  protoplasm, 
by  reason  both  of  its  decrease  and  of  its  intimate  relations  with  the 
surrounding  tissues;  the  cells  exhibiting  motion  in  the  adult  condition 
are  those  which  retain,  to  a  certain  degree  at  least,  their  embryonic 
type:  such  are  the  lymphoid  and  connective-tissue  cells. 

Motion  may  be  exhibited  by  elements  devoid  of,  as  well  as  by  those 
provided  with,  special  appendages.  The  lowest  degree  of  this  vital 
manifestation  is  encountered  in  the  streaming  of  the  protoplasm 
within  cells,  as  in  plants,  enclosed  within  limiting  membranes  which 
do  not  permit  such  motion  to  affect  the  exterior  of  the  cells.  Con- 
spicuous examples  of  the  more  marked  effects  of  protoplasmic 
streaming  are  familiar  in  the  changes  readily  observed  in  amcebae  or 
in  the  colorless  blood-cells  of  higher  types.  In  these  latter  elements, 
however,  the  motion  is  manifested  rather  in  change  of  form  than  by 
marked  variation  in  position. 

The  highest  expression  of  motion  is  displayed  by  those  cells  whose 
protoplasm  has  undergone  specialization,  resulting  either  in  the  pro- 
duction of  a  peculiar  tissue,  as  that  of  the  voluntary  muscle  fibre,  or 


22 


NORMAL   HISTOLOGY. 


of  external  appendages,  as  the  cilia  of  many  unicellular  organisms 
or  of  the  epithelial  elements  of  the  higher  animals. 

Since  every  cell  is  derived  from  a  pre-existing  cell,  it  follows  that 
all  the  cells  of  the  organism  are  the  descendants  of  the  parent  ele- 
ment— the  ovum.  The  ripe  mammalian  egg,  while  small  in  com- 
parison with  many  other  ova,  is  among  the  largest  histological 
elements,  measuring  about  .2  millimetre  in  diameter,  and,  further, 
possessing  all  parts  of  the  typical  cell. 

Before  the  ovum  is  capable  of  uniting  with  the  male  sexual  element 
to  carry  out  the  changes  attendant  upon  fecundation,  it  passes  through 
a  cycle  of  preparatory  stages  collectively  known  as  maturation. 
These  changes  consist  in  the  repeated  very  unequal  division  of  the 
ovum,  resulting  in  the  expulsion  of  minute  portions  of  its  proto- 
plasm, the  polar  bodies  ;  of  these  latter,  usually  two  are  extruded. 


Maturation  and  fecundation  in  ova  oiascaris  megalocephala  :  I,  «,  nucleus  of  ovum  before  matura- 
tion ;  s,  entering  spermatozoon  ;  //,  nucleus  («)  has  passed  to  periphery  of  cell  preparatory  to  di- 
viding; s,  spermatozoon  now  within  the  ovum  ;  ///,  nucleus  dividing  into  first  polar  body  (/)  ;  m, 
male  pronucleus  resulting  from  spermatozoon  ;  !V,p,p' ',  first  and  second  polar  bodies,  the  last  still 
in  process  of  formation  ;  m,  male  pronucleus  ;  V,  p,  p' ,  polar  bodies  ;  /  and  m,  respectively  female 
and  male  pronuclei,  in  contact  but  not  yet  fused  ;  c,  centrosomes,  indicating  poles  of  nuclear  spindle  ; 
VI,  pronuclei  now  fused  ;  striation  proceeds  from  centrosomes  preparatory  to  division  of  ovum.  (After 
O.  Hertwig.) 

The  nucleus  which  appears  within  the  ovum  after  the  formation  of 
the  polar  bodies  is  the  female  pronucleus.  Upon  the  completion 
of  these  phenomena,  maturation  has  taken  place  and  the  ovum  is 
prepared  for  the  reception  of  the  male   sexual   element.      Under 


THE   CELL   AND    THE   TISSUES.  23 

favorable  conditions  the  spermatozoa  reach  the  ovum,  when  a  single 
element  penetrates  the  envelopes  of  the  egg  and  is  received  within 
the  protoplasm  of  the  female  cell.  The  entrance  of  the  spermatozoon 
causes  a  new  disturbance  within  the  ovum,  resulting  in  the  formation 
of  the  male  pronucleus.  Subsequently  the  latter  joins  with  the 
female  pronucleus,  the  fusion  of  the  two  pronuclei  being  followed  by 
a  temporary  disappearance  of  all  nucleus  within  the  ovum.  Shortly 
afterwards  the  new  nucleus  of  segmentation  appears,  so  called 
from  the  fact  that  within  this  body  cleavage  of  the  ovum  is  first 
established. 

The  process  of  segmentation  following  the  fertilization  of  the 
ovum  is  essentially  one  of  indirect  cell-division,  in  which  the 
stages,  although  modified  in  certain  details,  are  essentially  the  same 
as  those  already  described.  The  mammalian  ovum  undergoes  a 
total  segmentation;  although  the  resulting  segments  are,  strictly 
regarded,  not  quite  equal  in  size,  yet,  as  a  matter  of  simplicity,  they 
may  be  regarded  as  such,  and  the  division  characterized  as  total 
equal  segmentation. 

The  repeated  cleavage  of  the  segmentation-spheres  into  which 
the  ovum  is  divided  soon  produces  a  mass  of  innumerable  cells  con- 
stituting the  blastoderm ;  the  latter,  by  continued  division  and 
further  differentiation,  subsequently  gives  place  to  a  cell-area,  in 
which  at  first  two  layers,  an  outer  and  an  inner,  and  later  a  third 
middle  stratum,  of  cells  appear.  These  more  or  less  imperfectly 
defined  tracts  constitute  the  important  primary  blastodermic 
layers,  the  ectoderm,  mesoderm,  and  entoderm,  from  which  are 
derived  all  the  tissues  of  the  body.  The  reader  must  be  referred  to 
the  various  text-books  of  embryology  for  a  detailed  account  of  the 
complicated  and  often  obscure  processes  of  maturation,  fertilization, 
segmentation,  and  blastulation,  of  which  only  the  most  salient  points 
have  been  indicated  above. 

THE   TISSUES. 

Every  tissue  is  composed  of  two  parts, — the  cellular  elements  and 
the  intercellular  substance.  Upon  the  first  of  these  depends  the 
vitality  of  the  tissue,  while  its  physical  properties  are  determined  by 
the  character  of  the  second.  The  physical  condition  of  the  inter- 
cellular substances  includes  a  wide  latitude,  varying  from  that  of  a 
fluid,  as  blood  or  lymph,  through  all  degrees  of  density,  until,  by 
the  additional  impregnation  of  calcareous  matters,  the  well-known 
hardness  of  bone  or  dentine  is  attained. 

The  proportion  between  the  cellular  elements  and  the  intercellular 
substance  of  mesodermic  tissues  varies  with  age  and  development, 
the  intercellular  substance  in  the  early  stages  being  scanty  and  very 


24 


NORMAL    HISTOLOGY. 


yielding,  while  with  adolescence  they  may  become  tough  and  re- 
sistant. Accompanying  the  growth  of  the  tissue,  an  increase  of  the 
intercellular  substance  usually  takes  place  through  the  direct  or 
indirect  participation  of  the  cells,  these  latter,  in  consequence,  suf- 
fering marked  reduction  in  number  and  size.  The  younger  the 
mesodermic  tissue,  the  richer  is  it  in  cells  and  the  poorer  in  intercellu- 
lar substances;  conversely,  the  older  the  tissue,  the  more  prominent 
the  intercellular  substance  and  less  conspicuous  the  cellular  elements. 
A  marked  example  of  this  law  is  presented  by  tendon,  where,  in  the 
embryonic  condition,  the  cells  constitute  the  greater  bulk  of  the 
tissue,  while  in  the  adult  the  intercellular  fibrous  tissue  so  overwhelms 
the  cellular  elements  that  reagents  are  frequently  necessary  to  satis- 
factorily demonstrate  their  existence. 

While  increase  of  the  intercellular  substance  usually  accompanies 
the  growth  of  the  mesodermic  tissues,  those  derived  from  the 
ecto-  and  entoderm  present  a  marked  contrast.  In  these  latter 
tissues  the  intercellular  constituent  is  represented  by  the  very  scanty 
cement  substance,  increase  in  which  occurs  only  as  necessitated  by 
the  growth  of  the  surrounding  cells,  the  proportion  between  the  two 
elements  being  practically  constant  throughout  life.  Instances  of 
this  constant  relation  are  seen  in  the  varieties  and  modifications  of 
the  epithelial  tissues. 

The  primary  blastodermic  layers — ectoderm,  mesoderm,  and 
entoderm — early  exhibit  histological  differences  which  suffice  to 
distinguish  the  one  from  the  other,  and  especially  to  indicate,  at  least 
in  a  general  manner,  the  tendency  of  the  outer  and  inner  layers  to 


Fig.  15. 


Blastodermic  layers  of  rabbit  embryo :  a,  ectoderm ;  b,  entoderm ;  c,  entodermal  cells  destined  to 
form  notochord  ;  m,  mesoderm. 


form  epithelial  structures  in  contrast  to  the  less  compact  and  more 
reticular  formations  of  the  mesoderm.  The  epithelia  of  the  genito- 
urinary tract,  however,  are  marked  exceptions  in  their  origin,  being 
derived,  as  well  as  the  connective  and  muscular  tissues,  from  the 
mesoderm,  in  this  respect  constituting  conspicuous  specializations. 


THE   CELL   AND   THE   TISSUES. 


25 


Derivatives  of  the  Primary  Blastodermic  Layers. 

From  the  ectoderm  are  derived — 

The  epithelium  of  the  outer  surface  of  the  body,  including  that 
of  the  conjunctiva  and  anterior  surface  of  the  cornea,  the 
external  auditory  canal,  together  with  the  epithelial  append- 
ages of  the  skin,  as  hair,  nails,  sebaceous  and  sweat  glands 
(including  the  involuntary  muscle  of  the  latter). 

The  epithelium  of  the  nasal  tract,  with  its  glands,  as  well  as  of 
the  cavities  communicating  therewith. 

The  epithelium  of  the  mouth  and  of  the  salivary  and  other 
glands  opening  into  the  oral  cavity. 

The  enamel  of  the  teeth. 

The  tissues  of  the  nervous  system. 

The  retina;  the  crystalline  lens. 

The  epithelium  of  the  membranous  labyrinth. 

The  epithelium  of  the  pituitary  and  pineal  bodies. 

From  the  mesoderm  are  derived — 

The  connective  tissues,  including  areolar  tissue,  tendon,  cartilage, 

bone,  dentine  of  the  teeth. 
The  muscular  tissues,  with  the  exception  of  the  muscle  of  the 

sweat-glands. 
The  tissues  of  the  vascular  and  lymphatic  systems,  including 

their  endothelium  and  circulating  cells. 
The  sexual  glands  and  their  excretory  passages,  as  far  as  the 

termination  of  the  ejaculatory  ducts  and  vagina. 
The  kidney  and  ureter  (but  not  the  bladder). 

From  the  entoderm  are  derived — 

The  epithelium  of  the  digestive  tract,  with  that  of  all  glandular 

appendages  except  those  portions  derived  from  ectodermic 

origin  at  the  beginning  (oral  cavity)  and  termination  of  the 

tube. 
The  epithelium  of  the  respiratory  tract. 
The  epithelium  of  the  urinary  bladder  and  urethra. 
The  epithelium  of  the  thyroid  and  thymus  bodies,  the  atrophic 

primary  epithelium  of  the  latter  being  represented  by  Hassall's 

corpuscles. 


26  NORMAL   HISTOLOGY. 


CHAPTER    II. 

THE    EPITHELIAL   TISSUES. 

The  free  surface  of  the  skin  and  of  the  various  mucous  membranes 
is  covered  by  epithelium,  which  affords  protection  to  the  more 
delicate  parts  lying  beneath.  In  this  tissue  the  intercellular  con- 
stituent is  reduced  to  a  minimum,  being  represented  alone  by  the 
scanty  cement-substance  between  the  cells;  the  latter,  in  consequence 
of  this  relation,  form  practically  an  unbroken  sheet. 

The  epithelia  are  best  grouped  under  two  chief  heads — squamous 
and  columnar.  The  designation  as  tessellated  or  pavement  is  not 
distinctive,  since  either  variety  may  present  a  mosaic  when  viewed 
from  the  free  surface.  These  tissues  may  be  classified  in  several 
divisions  as  below  indicated. 

VARIETIES   OF   EPITHELIUM. 

/   Sqziamous.  II.    Columnar. 

a.  Simple — consisting  of  a  single  layer — a.  Simple. 
b.  Stratified — consisting  of  several  layers — b.  Stratified. 

III.   Modified. 
a.  Ciliated;  b.  Goblet;  c.  Pigmented. 

IV.  Specialized, 
a.  Glandular  epithelium;  b.  Neuro-epithelium. 

The  epithelium  contains  no  blood-vessels,  the  nutrition  of  the 
tissue  being  maintained  by  the  absorption  of  the  nutritive  juices 
conveyed  by  means  of  the  intercellular  clefts  within  the  cement- 
substance.  The  nervous  supply  of  epithelium  is  likewise  ordinarily 
very  scanty,  the  existence  of  nerve-fibrils  within  the  epithelium  in 
many  localities  being  doubtful;  in  certain  regions  possessed  of  high 
sensibility,  as  the  corneal  or  tactile  surfaces,  the  termination  of  nerve- 
fibres  among  the  epithelial  elements  may  be  regarded  as  definitely 
established.  The  epithelial  cells  usually  rest  upon  a  basement- 
membrane,  or  membrana  propria,  a  modification  of  the  subjacent 
connective  tissue  of  which  it  is  part. 

The  principal  distributions  of  the  various  forms  of  epithelium 
follow. 


THE    EPITHELIAL   TISSUES. 


27 


Simple  squamous  epithelium  occurs  in  but  few  places: 

Partially  lining  the  tympanic  cavity,  including  the  mastoid 
cells ;  parts  of  the  membranous  labyrinth ;  the  infun- 
dibula  and  alveoli  of  the  lungs ;  the  posterior  surface  of 
the  anterior  capsule  of  the  crystalline  lens ;  parts  of 
ducts  of  glands  ;  the  capsule  of  the  Malpighian  body  and 
the  descending  limb  of  Henle'  s  loop  in  the  kidney;  choroid 
plexuses  and  parts  of  brain-ventricles. 

Stratified  squamous  epithelium  occurs  widely  distributed,  cover- 
ing— 

The  skin  and  its  extensions,  as  the  external  auditory  canal, 
conjunctival  sac,  and  cornea;  the  mouth,  lower  part  of 
pharynx,  and  oesophagus ;  the  epiglottis  and  upper  part 
of  larynx,  together  with  the  false  and  true  vocal  cords ; 
the  pelvis  of  kidney,  ureter,  bladder,  beginning  and  end 
of  male  and  entire  female  urethra;  the  vagina. 

Simple  columnar  epithelium  occurs: 

a.  Non-ciliated,  in — 

The  digestive  tract,  from  the  oesophageal  opening  of  stomach 
to  anus,  as  well  as  in  the  larger  ducts  of  the  glands  com- 
municating with  this  tube;  ducts  of  mammary  glands; 
seminal  vesicles  and  ejaculatory  ducts;  membranous  and 
penile  portions  of  urethra. 

b.  Ciliated,  in — 

Oviduct,  uterus,  and  part  of  canal  of  cervix ;  greater  part 
of  brain-ventricles  and  canal  of  spinal  cord. 

Stratified  columnar  epithelium  occurs  : 

a.  Non-ciliated,  in- 

Terminal  part  of  the  vas  deferens ;  olfactory  part  of  nasal 
fossae. 

b.  Ciliated,  in — 

The  Eustachian  tube  and  parts  of  tympanic  cavity ;  lachry- 
mal passages  ;  respiratory  part  of  nasal  fossae,  with  com- 
municating sinuses ;  ventricle  of  larynx,  trachea,  and 
bronchiae ;  epididymis  and  first  part  of  vas  deferens. 

Squamous  Epithelium.  When  occurring  as  a  simple  layer, 
the  flattened,  polyhedral,  nucleated  plates  form  a  regular  mosaic  ; 
such  epithelium  is  found  but  seldom  in  the  human  body,  the  lining  of 
the  air-sacs  of  the  lung,  the  posterior  surface  of  the  anterior  capsule 
of  the  crystalline  lens,  the  membranous  labyrinth,  and  a  few  other 
localities  being  its  principal  seats. 


28 


NORMAL   HISTOLOGY. 


Fig.  16. 


A  far  more  usual  arrangement  is  as  several  layers,  constituting 

the  stratified  squamous  variety.  The  isolated  cells  of  such  epi- 
thelium differ  greatly  in  form,  size,  and 
appearance  according  to  the  layer  from 
which  they  are  taken.  The  cells  com- 
posing the  deepest  stratum  are  not  scaly, 
but  irregularly  columnar,  resting,  with 
slightly  expanded  bases,  upon  the  sub- 
jacent membrana  propria.  The  irregular 
borders  of  these  cells  join  with  neighboring 
elements  in  such  a  manner  that  minute 
intercellular  clefts  are  formed ;  these  are 
occupied  by  the  yielding  cement-sub- 
stance, and  allow  the  passage  of  the 
nutrient  juices,  as  well  as  of  the  migratory 
leucocytes,     or    wandering    cells.       The 

nuclei  of  the  columnar  elements  are  oval,  and  often  situated  nearer 

the  outer  ends  of  the  cells. 

Passing  from  the  basement-membrane  towards  the  free  surface, 

the  form  of  the  cells  undergoes  a  radical  change.     The  pronounced 

columnar    type    belongs    to    the 

deepest  layer  alone ;  the  cells  next 

become      irregularly     polyhedral, 

Fig.  17. 


Squamous    epithelium    from    frog's 
skin,  viewed  from  the  free  surface. 


Fig.  18. 


Stratified  squamous  epithelium  in  section,  from 
the  cornea  :  the  deepest  cells  are  columnar  ;  the 
superficial  are  scaly  plates. 


Isolated  cells  of  stratified  squamous  epithelium  : 
a,  surface-cell ;  b  and  c,  cells  from  middle  layers  ; 
d,  from  deepest  stratum. 


then  gradually  expand  in  the  direction  parallel  to  the  free  surface,  and 
become,  finally,  converted  into  the  large  thin  scales  so  characteristic 
of  the  outer  layers  of  stratified  squamous  epithelium. 

The  cells  constituting  the  middle  strata  are  irregularly  polyhedral, 
and  not  infrequently  seem  to  be  mutually  connected  by  means  of 
delicate  processes,  which  bridge  the  intervening  intercellular  clefts 
and  establish  a  direct  continuity  between  the  neighboring  cells ;  when 
such  elements  are  isolated,  the  delicate  threads  are  broken  and  the 
disassociated  cells  appear  as  if  beset  with  minute  spines  :  these  con- 


Fig.  19. 


Prickle  -  cells  from 
middle  strata  of  the 
epidermis. 


THE   EPITHELIAL   TISSUES.  2Q 

stitute  the  prickle -cells.  During  the  journey  to  the  free  surface 
the  character  of  the  protoplasm  also  alters,  the  cells  losing  in 
vitality  and  becoming  keratose  or  horny  to  a 
greater  or  less  degree.  The  extent  to  which  these 
changes  occur  depends  upon  the  external  conditions 
affecting  the  tissue :  on  mucous  surfaces  kept  con- 
tinually moist  by  secretions  the  cells  retain  their 
plasticity  and  nuclei ;  where,  on  the  contrary,  they 
are  exposed  to  the  desiccating  influences  of  the 
atmosphere,  they  lose  their  nuclei  and  become  dry 
and  horny,  as  conspicuously  seen  in  the  superficial 
cells  of  the  epidermis.  Fatty  granules  and  small 
oil-drops,  sometimes,  also,  adherent  masses  of  bacteria,  are  common 
in  the  superficial  cells.  As  the  young  growing  cells  of  the  deeper 
layers  increase  in  size  and  numbers,  they  push  those  of  the  super- 
imposed strata  towards  the  free  surface,  where  the  older  superficial 
cells  become  loosened  and  gradually  set  free,  constituting  the  physio- 
logical desquamation  continually  taking  place. 

In  certain  localities,  as  in  the  urinary  bladder,  the  columnar  cells 
of  the  deep  layer  rapidly  assume  the  scaly  character  of  the  superficial 
strata ;  such  epithelium  possesses  relatively  few  layers,  and,  from  the 
facility  with  which  the  type  of  the  cells  changes,  is  often  described 
as  "transitional."  It  is  to  be  remembered  that  such  epithelium 
constitutes  not  a  distinct  variety,  but  only  a  modification  of  the 
stratified  scaly  group. 

Columnar  Epithelium.  The  columnar  epithelium,  when  occur- 
ring as  a  single  layer  of  cells,  constitutes  the  simple  columnar 
variety,  which,  however,  enjoys  a  much  wider  distribution  than  the 
corresponding  squamous  group.  The  taller  or  shorted  columnar 
cells  rest  upon  the  membrana 
propria  with  their  bases,  and 
join  their  neighbors  with 
more  or  less  accuracy.  The 
free  or  outer  ends  of  the  cells 
in  some  localities,  as,  conspicu- 
ously, in  the  intestine,  are  char- 
acterized by  the  presence  of  a 
narrow  marginal  zone,  or 
basal  border :  this  exhibits  a 
vertical  striation,  which,  on  the 
addition  of  a  reagent,  as  water, 
often  breaks  up  into  a  series  of 
rods,  resembling  very  robust  cilia.  When  the  single  layer  of  these 
epithelial  cells  is  replaced  by  several,  as  in  the  stratified  columnar 


Fig.  21. 


Fig.  20 


Simple  columnar  epi- 
thelium from  intestine : 
the  free  ends  of  the 
cells  present  a  peculiar 
striated  border- zone. 
Highly  magnified. 


Stratified  columnar  epi- 
thelium from  vas  defer- 
ens :  the  deepest  layer 
consists  of  small  cells, 
between  which  the  co- 
lumnar cells  extend. 


,Q  NORMAL   HISTOLOGY. 

variety,  the  outermost  cells  alone  are  distinctly  columnar ;  these  are 
usually  modified  at  their  outer  ends,  becoming  pointed,  forked,  or 
club-shaped,  in  order  to  fit  between  the  irregularly  polyhedral  and 
pyriform  elements  of  the  deeper  strata.  The  nucleus  is  situated  about 
the  middle  of  the  columnar  surface  cells,  and  somewhat  eccentrically 
nearer  the  basement-membrane  in  the  deeper  cells.  The  protoplasm 
of  columnar  epithelium  often  contains  particles  of  mucous  secretion, 
indicating  the  beginning  of  those  changes  which  result  in  the  produc- 
tion of  the  goblet-cells. 

Modified  Epithelium.  The  free  surfaces  of  the  epithelium  in 
particular  localities,  as  noted  in  detail  in  the  foregoing  summary, 
are  armed  with  minute  hair-like  processes,  or  cilia ;  these,  by  their 

constant  active  vibration,  create 


Fig.  22. 


Fig.  23. 


Ciliated  epithelium 
from  trachea  :  g,  a  cell 
filled  with  mucus  about 
to  be  discharged. 


Isolated  elements  of 
ciliated  columnar  epi- 
thelium from  trachea  : 
o,  tn,  i,  cells  from  sur- 
face, middle,  and  deep- 
est strata. 


a  current,  which  serves  to  free 
the  mucous  membranes  from 
accumulation  of  mucus  and  of- 
fending foreign  or  irritating  sub- 
stances. Cilia  are  specializations 
of  the  protoplasm,  with  which 
they  are  probably  directly  and 
intimately  connected  ;  widely  dis- 
tributed and  attached  to  the 
various  forms  of  epithelium  in 
the  lower  animals,  in  man  and  the 
higher  mammals  cilia  are  limited 
to  columnar  cells. 

The  exact  number  of  individual 
cilia  attached  to  the  free  surface 
of  a  single  cell  varies,  but  there  are,  probably,  between  one  and  two 
dozen  such  appendages  usually  present.  Their  length  likewise 
differs  with  locality,  those  lining  the  human  epididymis  being  about 
ten  times  longer  than  those  of  the  trachea.  When  analyzed,  by 
careful  observation  of  favorable  cells  in  not  too  rapid  vibration,  the 
motion  will  be  seen  to  consist  of  two  parts — a  rapid  primary  move- 
ment, directed  to  correspond  with  the  general  current,  and  a  slower 
secondary  return  to  the  original  position,  the  free  end  of  the  cilium 
describing  a  course  resembling  that  of  a  whip-lash.  The  vibrations, 
whose  rate  has  been  estimated  at  about  ten  per  second,  do  not  occur 
simultaneously  in  all  the  cells,  but  exhibit  a  progression,  one  cell 
after  the  other  taking  part  in  the  motions,  whereby  a  series  of  distinct 
waves  of  ciliary  motion  is  produced  ;  in  addition,  a  certain  periodicity 
or  rhythm  often  characterizes  the  vibrations. 

When  favorable  conditions  obtain,  including  a  sufficient  supply  of 
moisture,  oxygen,  and  heat,  ciliary  motion  may  be  maintained  for 


THE   EPITHELIAL   TISSUES. 


31 


Fig 


many  hours,  and  even  for  days ;  the  cells  of  cold-blooded  animals  in 
general  continue  to  vibrate  longer  than  those  of  mammals. 

The  rapidity  of  the  ciliary  motion  is  readily  influenced  by  tem- 
perature and  reagents.  While  the  application  of  gentle  heat  stimu- 
lates, the  motion  is  temporarily  arrested  by  a  reduction  to  50  C,  and 
permanently  impaired  by  an  elevation  above  500  C.  Increased 
motion  is  at  first  produced  by  the  addition  of  weak  alkalies  or  acids, 
followed,  however,  by  a  permanent  suspension  after  the  prolonged 
action  of  these  reagents.  Cold,  chloroform,  etc.,  on  the  contrary, 
effect  a  prompt  reduction  and,  finally,  stoppage  of  the  vibrations. 

On  surfaces  clothed  with  columnar  epithelium,  certain  cells  are 
distinguished  by  unusually  clear  protoplasm  and  exceptional  size; 
these  are  the  goblet-cells,  whose  peculiar  elliptical  or  chalice  form 
results  from  the  accumulation  of  mucoid  substance 
elaborated  within  their  protoplasm.  When  the  dis- 
tention becomes  too  great,  the  cell  bursts  in  the 
direction  of  least  resistance,  evidently  towards  the 
free  surface,  and  the  secretion  is  poured  out  on  the 
surface  of  the  mucous  membrane.  Goblet^cells 
occuron^all  surfaces  covered  by  columnar  ejpjthj^ljum, 
but  with  especial  profusion  in  the  large  intestine. 
These  elements  may  be  regarded  as  corresponding 
to  the  unicellular  glands  of  the  lower  animals  ;  in 
the  large  mucous  glands,  as  the  mucous  acini  of  the 
submaxillary  and  sublingual,  the  majority  of  the  secreting  elements 
are  in  a  condition  similar  to  that  of  the  goblet-cells. 

The  protoplasm  of  epithelial  cells  often  becomes  invaded  by  par- 
ticles of  foreign  substances ;  thus,  granules  of  fatty  and  proteid 
matters  are  very  commonly  encountered,  while 
the  presence  of  granules  of  eleidin  in  certain 
cells  of  the  epidermis  characterizes  the  stratum 
granulosum.  When  these  invading  particles  are 
colored,  asAvhen  composed  of  melanin,  the  pro- 
toplasm of  the  affected  cell  acquires  a  brown  or 
black  tint,  and  is  then  known  as  pigmented 
epithelium  ;  such  cells  are  constant  in  the  deeper 
layers  of  the  epidermis,  especially  of  certain  races, 
and  in  the  outer  layer  of  the  retina. 

Specialized    Epithelium.      Reference    has 
been  made  to  the  goblet-cells  as  being,  tempo- 
rarily at  least,  sufficiently  specialized  to  represent 
unicellular  glands ;  when  the  elements  become  permanently  modified 
to  engage  in  the  elaboration  of  secretion  they  are  recognized  as 
glandular  epithelium. 


Goblet  -  cells  from 
large  intestine  con- 
taining mucous  secre- 
tion. 


Fig.  25. 


Pigmented  epithelium 
from  outer  layer  of  ret- 
ina :  the  nuclei  (n)  still 
uninvaded. 


32 


NORMAL   HISTOLOGY. 


The  cells  lining  the  ultimate  divisions  of  glands  are  the  modified 
extensions  of  the  epithelial  investment  of  the  adjacent  mucous 
membrane,  of  which  they  are  the  direct  outgrowths.  Glandular 
epithelium  varies  in  form  from  columnar  (pancreas)  to  spherical 
(parotid)  and  polyhedral  (liver).  The  protoplasm  of  such  cells  is 
generally  more  or  less  filled  with  particles  of  secretion,  upon  whose 
quantity  and  arrangement  the  apparent  condition  of  the  protoplasm 
largely  depends.  Sometimes  the  latter  is  almost  entirely  displaced 
by  fatty  matters,  as  in  the  sebaceous  glands  or  in  the  active  mammary 
acini,  or,  again,  is  so  encroached  upon  by  particles  of  secretion  that  a 
reticulation  of  the  protoplasm  is  very  conspicuous. 

The  elements  lining  parts  of  certain  glands  exhibit  more  or  less  stria- 
tion,  on  account  of  which  peculiarity  such  cells  are  known  as  rod- epi- 
thelium ;  examples  of  this  are  seen  in  the  ducts  of  the  salivary  glands, 
and  in  the  irregular  and,  to  a  less  evident  degree,  the  convoluted 
portions  of  the  uriniferous  tubules  of  the  kidney. 
The  epithelial  coverings  of  those  areas  towards 
which  the  terminations  of  the  nerves  of  special 
sense   are   particularly  directed   undergo   high 

Fig.  27. 


Glandular  epithelium : 
small  acinus  from  a  serous 
racemose  gland. 


Rod-epithelium  :  a,  b,  c,  isolated  epithelial  cells  from  uriniferous 
tubules  of  rat  (after  Heidenhahi)  ;  d,  rod-epithelium  from  submax- 
illary duct  of  dog.     (After  Schieffer decker.) 


specialization,  resulting  in  the  production  of  perceptive  elements, 
to  which,  as  a  group,  the  name  neuro-epithelium  has  been 
applied.  The  rod-  and  cone-cells  of  the  retina, 
the  hair-cells  of  Corti's  organ  and  other  parts 
of  the  membranous  labyrinth,  the  olfactory  cells 
of  the  nasal  fossae,  and  the  taste-cells  of  the 
taste-buds,  are  all  familiar  examples  of  such 
specialized  epithelium.  In  these  elements  two 
parts  are  present — an  inner,  containing  the 
nucleus,  and  corresponding  to  the  usual  proto- 
plasm of  the  cells,  and  an  outer,  peripherally- 
directed  segment,  which  is  highly  specialized, 
and  not  infrequently  terminates  in  stiff,  rigid, 
hair-like  processes.  The  outer  segment  re- 
ceives the  stimuli  from  external  impressions, 
while  the  inner,  centrally-directed,  segment  stands 
in  close  anatomical  relation  with  the  nerve-fibres. 


Isolated  neuro-epithe- 
lium from  nose  :  o,  olfac- 
tory cells ;  s,  sustentacular 
•elements. 


THE    EPITHELIAL    TISSUES. 


33 


ENDOTHELIUM. 

Although  endothelium  is  intimately  related  to  the  connective  ] 
tissues,  being  but  modifications  of  the  cells  of  this  group,  it  is  con- 
venient to  describe  this  tissue  in  the  present  place.  Endothelium 
forms  a  covering  of  the  free  surface  of  those  spaces  not  directly 
communicating  with  the  external  atmosphere,  including,  therefore, 
the  lining  of  the  various  serous  cavities,  as  the  pleura,  pericardium, 
and  peritoneum  (disregarding  the  communication  established  through 
the  oviduct),  of  the  synovial  surfaces  of  joints,  of  the  heart  and 
blood-vessels,  as  well  as  of  the 

numerous    lymphatic   spaces  FlG-  29- 

and  vessels.  These  cells 
occur  normally  as  a  single 
layer  of  thin,  irregularly  poly- 
hedral plates  of  variable  size 
and  of  great  delicacy ;  they 
possess  an  oval,  sometimes 
kidney-shaped,  nucleus;  they 
never  overlap,  and  usually 
unite  with  neighboring  cells 
by  serrated  and  tortuous  lines 
of  cement-substance.  The 
endothelial  plates  covering 
the  serous  membranes  are, 
in  general,  polyhedral,  re- 
sembling in  outline  the  simple 
scaly  epithelium;  those  lining 
the  blood-vessels  are  elon- 
gated, irregular  spindles,  while 

those  found  in  the  lymphatic  vessels  are  often   still 
metrical,  being  limited  by  very  tortuous  boundaries. 

For  the  satisfactory  study  of  endothelium  resource  to  silver 
staining  (see  Appendix)  must  be  had,  by  which  method  the  inter- 
cellular cement-substance  is  colored  deeply  brown  or  black,  appear- 
ing as  dark,  frequently-interrupted  boundary-lines.  In  such  prepara- 
tions the  points  of  union  common  to  several  cells  are  often  marked 
by  small,  deeply-stained  areas — the  stigmata,  or  pseudo-stomata. 
These  figures  are  regarded  by  some  as  minute  openings  filled  by 
silver-stained  albuminous  substances ;  according  to  Klein,  however, 
many  of  these  stigmata  are  the  protruding  stained  processes  of  con- 
nective-tissue cells.  In  addition  to  these  areas  of  questionable  import 
are  true  distinct  openings,  the  stomata,  which  establish  direct  com- 
munication with  the  adjacent  lymphatic  channels ;   the  diaphragm, 

3 


Endothelium  from  peritoneal  surface  of  diaphragm, 
stained  with  silver  :  n,  nucleus  of  endothelial  plate  ;  s, 
one  of  the  intercellular  clefts  or  stigmata. 


more  unsym- 


34 


NORMAL   HISTOLOGY. 


Fig.  30. 


and   especially  the  septum  separating  the  peritoneal   sac  from  the 
abdominal  lymph-cavity  of  the  frog,  exhibit  well  these  pores.     The 

larger  stomata  are  lined  by 
several  small  granular  guard- 
cells,  whose  expansion  and 
contraction  largely  influence 
the  size  of  the  openings. 

The  development  of 
epithelium  is  intimately 
associated  with  the  exten- 
sions of  the  great  ecto-  and 
entodermic  tracts,  since,  with 
the  exception  of  the  epithe- 
lium of  the  greater  part  of 
the  genito-urinary  organs, 
the  epithelia  are  the  direct 
descendants  of  the  outer  and 
inner  embryonic  layers.  The 
cells  lining  the  passages  con- 
nected with  the  sexual  glands, 
as  well  as  the  urinary  tract  as  far  as  the  bladder,  are  derived  from 
those  of  the  Wolffian  body  and  duct,  and  hence  have,  with  these 
latter,  a  common  mesoblastic  origin.  The  simple  arrangement  of 
the  cells  in  the  earlier  stages  gradually  gives  place  to  the  more  com- 
plex disposition  of  the  mature  tissue. 

The  development  of  endothelium  forms  part  of  the  history  of 
the  changes  taking  place  within  the  extensive  mesodermic  areas ; 
from  the  specialized  sheet,  or  mesothelium,  bounding  the  primary 
body-cavity  of  the  young  embryo,  the  endothelium  of  the  pleural, 
pericardial,  and  peritoneal  cavities  directly  descends,  while  the  lining 
cells  of  the  vascular  and  lymphatic  channels  trace  their  origin  to  the 
differentiation  of  certain  of  the  mesodermic  elements. 


Endothelium  from  the  septum  cisternae  of  frog,  stained 
with  silver :  a,  one  of  the  true  stomata,  lined  with 
guard-cells  ;  b,  intercellular  cleft ;  w,  nucleus. 


THE   CONNECTIVE   TISSUES. 


35 


CHAPTER    III. 

THE    CONNECTIVE   TISSUES. 

The  important  group  of  connective  substances — the  most  widely 
distributed  of  all  tissues — is  the  direct  product  of  the  great  meso- 
blastic  tract,  axial  as  well  as  peripheral ;  the  several  members  of  this 
extended  family  are  formed  by  the  differentiation  and  specialization 
of  the  intercellular  substance,  wrought  through  the  more  or  less 
direct  agency  of  the  mesoblastic  cells.  The  variation  in  the  physical 
characteristics  of  these  substances  is  due  to  the  condition  of  the 
intercellular  constituents  of  the  tissues.  Taken  during  the  period 
of  embryonal  growth,  they  are  represented  by  a  semi-gelatinous, 
soft,  plastic  mass ;  a  little  later,  the  still  soft,  but  already  definitely 
formed,  growing  connective  tissue  exists,  which  is  soon  replaced  by 
the  yielding,  though  strong,  adult  areolar  tissue.  Grouped  as  masses 
in  which  the  white  fibrous  tissue  predominates,  the  marked  tough- 
ness of  tendon  is  reached  ;  or  where  large  quantities  of  yellow  elastic 
tissue  are  present,  great  extensibility  is  secured.  A  further  conden- 
sation of  the  intercellular  substance  produces  the  resistance  of  the 
matrix  of  hyaline  cartilage,  with  the  intermediate  gradations  pre- 
sented by  the  fibrous  and  elastic  varieties ;  the  ground-substance 
becoming  additionally  impregnated  with  calcareous  salts,  the  well- 
known  hardness  of  bone  or  dentine  is  attained.  In  all  these  varia- 
tions in  the  density  of  the  intercellular  substance  the  cells  have 
undergone  but  little  change — the  connective-tissue  corpuscle,  the 
tendon-cell,  the  cartilage-cell,  and  the  bone-corpuscle  being  morpho- 
logically identical. 

The  principal  forms  in  which  connective  tissue  occurs  are, — 
i.   Mucous  tissue,  as  in  the  jelly  of  Wharton  of  the  umbilical  cord. 

2.  Growing,  immahire  tissue,  as  in  very  young  animals  or  in  old 
embryos. 

3.  Areolar  tissue,  as  in  the  subcutaneous  and  intermuscular  tissues. 

4.  Dense  mixed  fibrous  and  elastic  tissue,  as  in  the  sclera,  fasciae, 
etc. 

5.  De?ise  white  fibrous  tissue,  as  in  tendon,  cornea. 

6.  Dense  elastic  tissue,  as  in  the  ligamenta  subflava. 

7.  Cartilage — fibrous,  elastic,  and  hyaline  varieties. 

8.  Bone. 

9.  Dentine. 

10.    The  reticulum  of  adenoid  tissue. 


36 


NORMAL    HISTOLOGY. 


ii.    The  supporting  connective  tissue  of  the  nervous  system. 

1 2.  The  supporting  and  uniting  framework  of  the  various  organs. 

13.  Adipose  tissue. 

The  cellular  elements  of  the  connective  tissues  are  usually  de- 


scribed  as   of  two    kinds — the 
proper  and  the  migratory  or 

Fig.  31. 


Connective-tissue  cell  from  young  subcu- 
taneous tissue  :  w,  wing-like  expansion  seen 
in  profile. 


Fig.  32. 


"fixed"  or  connective-tissue   cells 
wandering"  cells.     The  former,  in 
their  typical  and  unrestrained  con- 
dition,  are   flattened   stellate    pro- 
toplasmic plates,  each  with  a  nu- 
cleus occupying  the  thicker  part  of 
the  body  of  the  cell,   from  which 
branched  processes  extend;  in  some 
instances  the  protoplasm  extends  in 
several   planes   as   thin,    plate-like 
wings.     The  nuclei  are  limited  by 
distinct  membranes,  and  frequently 
contain  well-marked  nucleoli. 
While  possessing  in  its  early  condition  the  plate-like  form  in  a 
greater  or  less  degree,  the  ordinary  connective-tissue  cell,  owing  to 
its  participation  in  the  formation  of  the  intercellular  tissue,  suffers 
greatly  during  the  later  stages  of  its  history;  the  expanded  cell-body 

soon  gives  place  to  smaller  outlines, 
while  the  protoplasm  diminishes  until 
the  once  large  element  is  reduced  to 
the  inconspicuous  spindle-cells  of 
adult  areolar  tissue,  in  which  only  a 
thin  envelope  of  protoplasm  sur- 
rounds the  nucleus.  The  connective- 
tissue  cells,  when  rich  in  protoplasm 
and  under  favorable  conditions,  are 
capable  of  exhibiting  amoeboid  move- 
ments, the  variations  being,  however, 
limited  to  alterations  of  form  brought 
about  by  the  extension  or  retraction 
of  the  protoplasmic  processes. 
Associated  with  the  flattened,  plate- 
like elements  of  connective  tissue,  in  many  places  are  found  the  highly- 
vacuolated  plasma-cells  of  Waldeyer.  These  are  of  uncertain  form, 
often  irregular,  extended,  or  spindle,  and  consist  of  soft  protoplasm, 
which,  owing  to  the  numerous  vacuoles  contained,  presents  an  appear- 
ance in  marked  distinction  to  that  of  the  ordinary  branched  cell. 
The  plasma-cells  probably  bear  a  somewhat  constant  relation  to 
young  tissues  in  which  the  formation  of  new  blood-vessels  is  still 
progressing. 


Embryonal  connective  tissue :  the  inter- 
cellular substance  is  only  slightly  differen- 
tiated. 


THE    CONNECTIVE   TISSUES. 


37 


In  addition,  occasional  peculiar  granule-cells  must  be  recognized. 
These  elements,  entirely  distinct  constituents  of  connective  tissues, 
often  appear  spherical  in  form,  and  are  distinguished  by  the  con- 
spicuous granularity  of  their  protoplasm,  the  granules  possessing  a 
strong  affinity  for  eosin  and  many  aniline  stains.  The  granule-cells 
occur  in  especial  pro- 
fusion in  the  vicinity  Fig.  3s. 
of  blood-vessels,  and 
seem  to  be  intimately 
connected  with  the 
formation  of  adipose 
tissue. 

In  contrast  to  these 
larger  connective-tis- 
sue elements,  irregu- 
larly round  or  ovoid 
smaller  cells  are  often 
present,  which,  from 
their  ability  to  change 
their  position  as  well 

as  form,  are  termed  the  wandering  cells.  These  consist  of  small, 
nucleated  masses  of  active  protoplasm,  characteristic  of  the  lymph  or 
colorless  blood-cells  with  which  they  are  identical,  usually  being 
really  leucocytes  which  have  passed  out  of  the  vessels  into  the 
surrounding  tissues,  through  which  they  wander  as  transient 
guests. 

The  protoplasm  of  the  fixed  cells  sometimes  exhibits  accumula- 
tions of  dark  particles,  the  elements  then  appearing  as  the  large, 
irregularly  branched 

Fig.  34. 


Subcutaneous  areolar  tissue  :  c,  c,  some  of  the  connective-tissue 
corpuscles  ;  iv,  migratory  cells  ;  v,  ptasma-cell  ;  e,  elastic  fibres. 


Fig.  35. 


pigment-cells, 
which  form  con- 
spicuous ob- 
jects in  the  con- 
nective tissues 
of  many  of  the 
lower  animals  ; 
in  man,  such 
cells  occur  prin- 
cipally     within 

the  choroid  and  iris,  and  in  certain  parts  of  the  pia  mater.  The 
pigment-cells  vary  in  shape  and  size ;  usually  stellate  and  of  mod- 
erate extent  in  the  higher  vertebrates,  they  assume  the  most  elabo- 
rate and  grotesque  forms  and  reach  enormous  dimensions  within  the 
tissues  of  the  lower  animals. 


Special  connective-tissue  elements  : 
p,  vacuolated  plasma-cells;  g,  granule- 
cells. 


Pigmented  connective-tissue  cor- 
puscles from  the  choroid. 


Pigment-cell  from  newt's  skin. 


-g  NORMAL   HISTOLOGY. 

The  immediate  vicinity  of  the  blood-vessels  is  a  favorite  locality 
for  pigment-cells,  their  arborescent  processes  often  forming  a  net- 
work completely  enclosing  the 
Fig.  36.  vessel.    The  supporting  stroma 

of  various  organs  of  many  of 
the  lower  animals  frequently 
contains  such  cells,  the  liver 
constantly  presenting  con- 
spicuous groups  of  deeply-pig- 
mented  elements.  Pigment- 
cells  are  capable  of  spontaneous 
movement,  the  changes  in- 
cluding not  only  alterations  or 
retractions  affecting  the  pro- 
cesses— phenomena  directly  influenced  by  the  action  of  the  light — 
but  likewise  decided  alterations  in  position  and  location  of  the  cells. 
The  granules  of  the  dark-brown  pigment  are  usually  regarded  as 
composed  of  melanin  derived  from  the  coloring-matters  of  the  blood; 
recent  investigations,  however,  render  it  probable  that,  while  appar- 
ently the  same,  the  dark  pigment  found  within  the  various  tissues  is  by 
no  means  always  identical  in  composition.  The  isolated  particles  when 
examined  with  high  amplification  are  but  slightly  colored,  the  charac- 
teristic tint  appearing  only  when  the  pigment-granules  are  massed. 
Whether  the  colored  particles  are  taken  up  by  the  cells  as  pre-existing 
pigment-granules,  or  whether  they  are  produced  within  the  proto- 
plasm of  the  cell,  is  still  undecided  ;  the  evidence,  however,  seems  to 
favor  the  conclusion  that  the  particles  possess  an  extra-cellular  origin. 
The  arrangement  of  the  connective-tissue  cells  varies  with 
the  age  and  density  of  the  tissue.  Where 
the  cells  retain  the 


Fig.  39. 


stellate  type,  a  pro- 
toplasmic net-work 
extending  through- 


Fig.  38. 


Fig.  37. 


Plate -like  connective- 
tissue  cells  found  in  ten- 
don. 


Cell-spaces  of  dense  con- 
nective tissue  in  which  the 
cells  lie :  silvered  ground- 
suhstance  :  from  the  cornea. 


Connective  -  tissue  (corneal)  corpus- 
cles :  these  cells  occupy  the  spaces 
within  the  ground-substance. 


out  the  tissue  is  formed  by  the  union  of  the  processes ;  examples  of 
such  disposition  are  seen  in  young  mucoid  tissues,  the  cornea,  and 
other  connective  substances  rich  in  cells.     Parallel  rows  of  closely- 


THE   CONNECTIVE   TISSUES. 


39 


placed  quadrate  elements  are  seen  in  tendon,  while  sheets  of  flattened 
endothelioid  plates  characterize  basement-membranes  and  envelop 
the  bundles  of  fibrous  tissue. 

In  the  denser  structures  the  cells  occupy  spaces  within  the 
ground-substance  ;  these  spaces  usually  communicate  directly  with 
one  another  by  means  of  minute  channels,  or  canaliculi,  and  form  a 
complicated  system  of  "juice-canals"  through  the  entire  tissue. 
Within  these  tissue-spaces,  or  lacun<z,  lie  the  connective-tissue 
corpuscles,  generally  only  partially  filling  the  cavities,  and  being 
usually  especially  applied  to  one  wall  of  the  space  after  the  manner 
of  an  endothelial  covering.  These  interfascicular  clefts  within  the 
ground-substance  may  be  regarded  as  the  radicles  of  the  lymphatic 
system,  in  some  localities,  as  in  the  peritoneum,  standing  in  close 
relation  with  both  the  lymphatic  and  the  blood  channels. 

The  intercellular  or  fibrous  constituents  of  connective  tissue  are  of 
two  kinds — white  fibrous  and  yellow  elastic  tissue.  White  fibrous 
tissue  ordinarily  occurs  as  wavy  bundles  of  varying  thickness,  com- 
posed of  silky  fibrils  of  such  fineness  that,  under  ordinary  amplifica- 
tion, they  present  no  appreciable  thickness  ;  these  bundles  sometimes 
run  parallel,  as  in  tendon,  but  more  frequently  interlace,  forming 
coarser  or  finer  mesh-works,  as  seen  in  the  omentum  and  subcu- 
taneous tissues.  When  examined  after  teasing,  the  ultimate  fibrils 
of  the  white  fibrous 
tissue    appear    as    a  Fig   — 

confused  mass  of 
delicate  interlacing 
lines,  but  in  their 
undisturbed  relation 
they  lie  parallel, 
whatever  may  be  the 
general  disposition  of 
the  bundles.  Fibrous 
tissue  yields  gela- 
tin on  boiling  in 
water,  and  swells  up 
and  becomes  in- 
distinct on  treat- 
ment with  acetic  acid. 

Yellow  elastic 
tissue,  on  the  con- 
trary, occurs  usually 
as  a  net-work  of  dis- 
tinct fibres  lying  among  the  bundles  of  the  white  fibrous  tissue. 
Examined  in  detail,  the  elastic  fibre  appears  highly  refracting  and 


White  fibrous  tissue : 
one  end  of  the  bundle  has 
been  teased  to  display  the 
component  fibrillae. 


Elastic  fibres  isolated  ;  from  the  ad- 
ventitia  of  the  aorta.  (After  Schief- 
ferdecker.) 


40 


NORMAL    HISTOLOGY. 


homogeneous,  and  possesses  a  definite  width  throughout  its  length, 
although  the  several  fibres  forming  the  same  net-work  may  vary  in 
thickness  ;  not  infrequently  slight  triangular  thickenings  are  found 
at  the  points  marking  the  union  of  several  fibrils.  Loosened  from 
their  attachments,  the  elastic  fibres  assume  a  wavy,  bent  or  coiled 
condition,  highly  characteristic.  Elastic  fibres  do  not  yield  gelatin 
when  boiled,  but  contain  elastin,  which  is  probably  enclosed  within 
a  sheath  of  great  delicacy,  but  of  considerable  resistance  towards 
reagents. 

The  most  immature,  and  morphologically  the  youngest,  form  of 
connective  tissue  is  mucous  tissue,  a  typical  example  of  which  is 
found  in  the  jelly  of  Wharton,  in  the  umbilical  cord.  Here  the 
stellate  cells  still  retain  their  embryonal  characters,  and,  by  the  union 
of  their  processes,  form  a  protoplasmic  net-work  throughout  the 
tissue  ;  the  meshes  of  this  net-work  are  occupied  by  a  semi-gelatinous, 
indifferent,  and  but  slightly  differentiated  intercellular  substance, 
containing  few  fibres  and  occasional  wandering  cells. 

All  gradations  of  density  between  the  immature  mucous  and  the 
more  resistant  areolar  tissue  are  supplied  by  the  various  stages  of 
development.  Ordinary  connective  or  areolar  tissue,  as  found  be- 
neath the  skin  and  in  many  other  localities,  comprises  both  white 
fibrous  and  elastic  tissue.     The  former  usually  occurs  as  wavy  bundles, 

which  interlace  to  form  a  felt-work  of 
varying  compactness ;  it  is  probable 
that  the  bundles  are  confined  by  a 
delicate  sheath,  strengthened  by  trans- 
versely and  spirally  wrapped  fibrils, 
whose  positions  are  marked  as  con- 
strictions, after  the  treatment  of  the 
bundles  with  acetic  acid.  The  indi- 
vidual fibrils  composing  the  bundles 
lie  embedded  within  and  held  together 
by  a  soft  homogeneous  ground-sub- 
stance, securely  uniting  them  ;  in  the 
denser  tissues  the  ground-substance 
contains  intercommunicating  cell- 
spaces  and  canaliculi,  the  surrounding  areas  appearing  as  a  homo- 
geneous matrix.  The  elastic  fibres,  in  varying  number  and  size, 
form  a  net-work  throughout  the  tissue.  The  fixed  connective-tissue 
corpuscles  lie  embedded  among  or  directly  applied  to  the  surface  of 
the  bundles  of  white  fibrous  tissue,  forming,  in  such  cases,  an  im- 
perfect wrapping  or  covering  ;  within  the  interfascicular  clefts  are  the 
wandering  cells. 

The  density  of  the  tissue  depends  largely  upon  the  amount  and 


Fig.  42. 


Connective-tissue  cells  from  young  urn- 
bilical  cord:  processes  of  cells  unite  to 
iirm  protoplasmic  net-work  ;  fibrous  ele- 
ments slightly  developed. 


THE    CONNECTIVE   TISSUES. 


41 


:V^ 


Peripheral  part  of  a  tendon  in  section  :  a,  external 
fibrous  investment  sending  partitions  between  the 
secondary  groups  (b)  of  the  tendon-fibres ;  the  small 
stellate  figures  represent  the  stained  contents  of  the 
interfascicular  clefts. 


arrangement  of  the  white  fibrous  element,  while  its  extensibility  is 
determined  by  the  proportion  of  elastic  tissue  present.  When  the 
former  occurs  in  well-defined 

bundles,  felted  together  into  Fig.  43. 

interlacing  lamellae,  dense  and 
resistant  structures  result,  as 
fasciae,  the  cornea,  etc. ;  in 
such  structures  the  cement- 
substance  within,  the  interfas- 
cicular clefts  is  usually  hol- 
lowed out  to  form  the  spaces 
occupied  by  the  connective- 
tissue  cells  and  their  pro- 
cesses. 

Tendon  represents  a  dense 
connective  tissue,  composed 
almost  entirely  of  white 
fibrous  tissue  arranged  in  parallel  bundles  of  varying  thickness. 
The  primary  bundles,  made  up  of  the  ultimate  fibrillar,  are  held 
together  to  form  larger  secondary  ones,  which  latter  are  enveloped 
in  a  delicate  sheath  covered  by  endothelial  plates ;  the  secondary 
bundles  are  bound  together  and  grouped  by  connective-tissue  septa, 
which  are  extensions  of  the  thick  external  sheath  wrapping  the 
entire  tendon.  The  larger  septa  support  the  blood  and  lymphatic 
vessels. 

The  flattened  connective-tissue  corpuscles,  or  tendon-cells,  occur 
in  rows  within  the  clefts,  between  the  primary  bundles,  upon  and 
between  which  the  thin,  plate- 
like bodies   and  wings   of  the  Fig.  44. 
tendon-cells     expand.       Seen 
from   the   surface,    these    cells 
appear    as    quadrate     bodies, 
whose  oval  nuclei  are  frequently 
so  disposed  that  those  of  two 
neighboring  cells  are  in  close 
proximity,  lying  near  the  ad- 
jacent ends  of  the  cells,  from 
which  arrangement   it  follows 
that  each  pair  of  nuclei  is  sep- 
arated by  the  greater  part  of  the  length  of  two  cells.     Viewed  in 
profile,    the   tendon-cells   show  as   narrow,    irregularly  rectangular 
bodies ;  while  when  examined  in  transverse  section  the  same  cells 
appear  as  stellate  bodies,  whose  extended  arms,  passing  often  in 
several  planes,  represent  the  sections  of  the  wing-plates.     Each  cell 


Primary  bundles  of  white  fibrous  (tendon)  tissue, 
on  and  between  which  the  flattened  tendon-cells  lie : 
at  j  these  are  seen  from  the  surface ;  at  o  and  p, 
oblique  and  profile  views. 


42 


NORMAL   HISTOLOGY. 


occupies  a  corresponding  space  within  the  cement-substance,  just  as 
do  the  cells  of  other  dense  forms  of  connective  substances.     Elastic 

fibres   are   almost,  if  not  en- 
Fig.  45.  tirely,  wanting  in  tendon. 

Elastic  tissue,  as  usually- 
encountered  as  an  element 
of  areolar  tissue,  occurs  in 
slender  fibres ;  where,  how- 
ever, the  elastic  tissue  be- 
comes the  dominating  con- 
stituent, as  in  the  ligamentum 
nuchas  or  ligamenta  subflava 
of  man,  the  fibres  assume 
much  greater  size,  becoming 
coarse  and  of  considerable 
diameter.  On  transverse  sec- 
tion of  such  tissue  the  robust 
individual  elastic  fibres  appear  as  irregularly  angular  or  polyhedral 
areas  ;  these  are  of  variable  size  and  held  together  by  a  small  quantity 
of  areolar  tissue.  The  fibres  of  elastic  tissue  may  become  broad  and 
flattened  out,  and  so  closely  placed  that  they  assume  the  form  of  a 
reticulated  elastic  membrane,  as  Henle 's  fenestrated  membrane  of 
the  larger  arteries ;  again,  the 
tissue  may  assume  the  form  of  a  f'g-  47- 

continuous  elastic  sheet,   as  Des- 
cemeV  s  -membrane  of  the  cornea. 

The     development     of     the 
white    fibrous   tissue    is   still  a 

Fig.  46. 


Primary  tendon-bundles  in  section  :  b,  the  tendon- 
tissue  ;  s,  interfascicular  clefts  occupied  by  granular 
material  and  the  tendon-cells  (a)  applied  to  the  bundles. 


Elastic  fibres  in  transverse  section  ; 
from  the  ligamentum  nuchae  :  a,  are- 
olar tissue  separating  the  groups  of 
the  elastic  fibres ;  b,  the  individual 
elastic  fibres  in  section. 


Elastic  fibres  closely  placed,  form- 
ing the  fenestrated  membrane  ;  from 
the  aorta. 


subject  of  much  uncertainty.  It  may  be  regarded,  however,  as 
established  that  it  is  through  the  agency  of  the  cells,  indirect  although 
their  influence  may  be,  that  the  fibres  of  connective  tissue  originate. 
Two  methods  are  recognized  in  the  production  of  the  fibres.  The 
doctrine  of  the  direct  mode  assumes  the  transformation  of  the  cell 


THE   CONNECTIVE   TISSUES. 


43 


protoplasm  into  the  white  fibrillae,  the  greatly  elongated  cell-body 
becoming  the  fibres.  While  such  conversion  does  probably  occur, 
it  is  certain  that  the  indirect  mode,  whereby  the  fibres  originate 
within  an  indifferent  matrix,  is  the  more  usual ;  the  production  of 
the  matrix  or  ground-substance  itself,  however,  must  be  attributed 
to  the  cellular  elements.  Regarding  the  development  of  the 
elastic  fibres,  strong  evidence  supports  the  view  that  the  fibres  are 
produced  without  the  direct  action  of  the  cells,  but  result  from  the 
fusion  of  longitudinally-disposed  rows  of  minute  particles,  which 
appear  within  the  indifferent  intercellular  matrix. 

Adipose  tissue  must  be  regarded  as  a  member  of  the  group  of 
connective  substances,  since  the  accumulation  of  oily  matters  within 
the  protoplasm  of  connective-tissue  cells  is  responsible  for  the  highly 

Fig.  48. 


Fat-cells  embedded  in  subcutaneous  areolar  tissue  :  f,  fat-cells  ;  «,  nucleus  ;  c, 
connective-tissue  corpuscles ;  iu,  migratory  cells ;  e,  elastic  fibres ;  b,  capillary 
blood-vessel. 

characteristic  appearance  of  the  tissue.  Whether  the  fat-cells  are 
developed  from  elements  especially  set  apart  for  this  role,  or  whether 
they  are  but  modified  ordinary  connective-tissue  cells,  is  still  a  dis- 
puted point ;  there  are,  however,  strong  reasons  for  holding  the  latter 
view  as  correct. 

Examined  after  the  usual  preparatory  manipulations,  and  in  places 
where  the  cells  maintain  their  individual  forms,  as  in  the  omentum, 
adipose  tissue  is  seen  to  be  made  up  of  relatively  large,  clear,  oval 
or  spherical  sacs.  The  transparent  contents  are  limited  by  a  delicate 
envelope,  composed  of  cell-membrane  and  an  extremely  thin  layer 
of  protoplasm  ;  on  one  side  of  the  sac  a  local  accumulation  marks 
the  position  of  the  nucleus. 

Fat-cells  occur  usually  in  groups,  supported  and  held  together  by 
areolar  tissue,  through  which  ramifies  a  rich,  vascular  net-work.  In 
localities  possessing  considerable  masses  of  adipose  tissue,  as  beneath 
the  scalp  and  the  skin,  the  cells  are  grouped  into  lobules,  and  these 


,,  NORMAL   HISTOLOGY. 

44 

again  Into  larger  masses,  or  lobes  ;  where  aggregated  and  closely- 
pressed  together,  the  normal  spherical  shape  of  the  individual  fat- 
sacs  gives  way  to  a  polyhedral  form. 

Adipose  tissue  possesses  a  rich  vascular  supply,  an  arteriole  passing 
to  each  lobule,  there  to  break  up  into  capillary  net-works,  which 
surround  the  individual  sacs.  The  development  of  adipose  tissue  is 
probably  not  confined  to  any  particular  kind  of  connective-tissue 
cell,  but  may  involve  any  of  the  corpuscles.  The  granule-cells, 
however,  seem  to  bear  a  close  relation  to  the  production  of  fat-tissue. 

In  those  elements  about  to  become  fat-cells,  a  few  oil-drops  appear 
within  the  protoplasm  ;  these  increase  in  size,  coalesce,  and  gradually 
encroach  upon  the  cell-contents,  pushing  the  nucleus  towards  the 
periphery.  This  displacement  progresses  with  the  increasing  volume 
of  the  accumulating  oil,  until,  finally,  the  once  slender  cell  is  trans- 
formed into  a  distended  vesicle,  whose  protoplasm  is  expanded  to  an 
almost  invisible  layer  immediately  beneath  the  cell-wall,  containing, 
at  one  side,  the  flattened  and  displaced  nucleus,  which  now  appears, 
in  profile,  as  an  attenuated  crescent.  Observations  on  starved  animals 
show  that  after  the  withdrawal  and  disappearance  of  the  fatty  matters, 
the  cells  are  capable  of  resuming  the  usual  appearance  and  properties 
of  connective-tissue  corpuscles. 

CARTILAGE. 

Cartilage  represents  a  dense  connective  tissue  in  which  the  inter- 
cellular substance  has  undergone  great  condensation.     Depending 

upon  the  variation  in  the 
FlG'  I9'  character  of  the   matrix 

between  the  cells,  three 
varieties  of  cartilage  are 
recognized  —  hyaline , 
elastic,  and  fibrous. 
Regarded  in  their  re- 
lationship to  the  denser 
connective  tissues,  the 
order  of  enumeration 
should  be  reversed,  the 
fibrous  variety  standing 
„    ,.  '       "        ,  '  '..      ,       „    ,.       ,    ,,  ,     next    and    differing-    but 

Hy.line   cartilage   from   the   rib:    the   cells   he   embedded  _  » 

wiihin  the   lacunae,  either  sing'y,   in   pairs,  or  in   groups;  little  fl'Om  tendon.       Since 

matrix  exhibits  differentiation  around  the  cell-spaces  as  more  u       ' '  portil'icrp' '    thp    tvni- 
ckeply  staining  areas.  ^  ,     **  .  . 

cal     hyaline     variety     is 
usually  understood,  that  form  first  claims  attention. 

Hyaline  cartilage,  so  named  from  the  transparent,  apparently 
homogeneous  character  of  the  intercellular  matrix,  enjoys  a  very 


THE    CONNECTIVE   TISSUES. 


45 


Fig.  50. 


wide  distribution,  occurring  as  the  articular  cartilage  of  bones,  costal 
cartilages,  the  larger  cartilages  of  the  larynx,  trachea,  or  bronchi, 
nose,  Eustachian  tube,  etc. ;  in  the  embryo  the  entire  skeleton,  with 
the  exception  of  the  vault  of  the  cranium,  the  bones  of  the  face,  and 
the  greater  part  of  the  lower  jaw,  is  mapped  out  by  primary  hyaline 
cartilage. 

The  homogeneity  of  the  hyaline  matrix  is  only  apparent,  since, 
as  long  ago  pointed  out  by  Leidy,  the  intercellular  substance  may  be 
resolved  into  bundles  of  fibrous  connective  tissue,  which,  however, 
are  so  closely  united  and  intimately  blended  by  the  cementing  ground- 
substance  that  the  presence  of  the  fibres  is,  ordinarily,  not  evident. 
After  prolonged  boiling,  cartilage  matrix  yields  chondrin. 

Embedded  within  the  hyaline  matrix  lie  the  cartilage-cells ; 
these  are  irregularly  oval  or  angular  nucleated  protoplasmic  bodies, 
which,  during  life,  almost  fill  the  spaces,  or  lacunae,  which  they 
occupy.  In  adult  tissue  usually  two  or 
more  cells  share  the  same  compartment, 
the  original  occupant  of  the  space  having 
undergone  division,  so  that  two,  four,  or 
even  more  daughter-cells  form  a  single 
group.  The  matrix  immediately  sur- 
rounding the  lacuna  is  specialized  as  a 
layer  of  different  density,  thereby  as- 
suming the  appearance  of  a  distinct  limit- 
ing membrane,  described  as  the  capsule. 
A  further  differentiation  of  the  ground- 
substance  is  seen  in  the  greater  intensity 
with  which  the  more  recently  formed 
matrix  enveloping  the  cells  stains;  such  re- 
sulting figures  constitute  the  cell-areas. 
It  is  to  be  remembered  that  the  cartilage- 
cells  are  but  connective-tissue  cells,  and 
that  the  lacunae  correspond  to  the  lymph- 
or  cell-spaces  found  in  other  dense 
connective  tissues.  Since  it  is  usual  to 
find  these  cell-spaces  in  communication 
through  minute  channels,  or  canaliculi, 
their  absence  and  the  apparent  isolation 

of  the  lacunae  in  cartilage  are  to  be  regarded  as  deviations  from  the 
typical  arrangement ;  among  some  lower  forms,  however,  such  a 
communication  exists,  the  minute  canaliculi  passing  between  the 
neighboring  lacunae. 

The  free  surface  of  the  cartilage  is  covered  by  an  envelope  of  dense 
connective  tissue,  the  perichondrium  ;   this  consists  of  an  external 


Hyaline  cartilage  with  perichon- 
drium (f)  attached  :  y,  zone  of 
youngest  cartilage-cells;  in,  hyaline 
matrix  enclosing  the  lacunas  contain- 
ing the  cartilage-cells  ;  /,  space  from 
which  the  cell  has  been  lost. 


.5  NORMAL   HISTOLOGY. 

or  fibrous  layer  of  dense  fibro-elastic  tissue  and  an  inner,  much 
looser  stratum,  between  the  fibres  of  which  are  numerous  connective- 
tissue  cells.  This  inner  portion  is  intimately  concerned  in  the  pro- 
duction of  new  cartilage,  and  is  known  as  the  chondrogenetic  layer. 
The  cells  of  the  latter  arrange  themselves  in  rows  parallel  to  the 
surface,  and  gradually  assume  the  characteristics  of  the  cartilage 
corpuscles,  being  at  first  spindle-shaped,  but  gradually  assuming  the 
more  spherical  form.  The  new  cells  soon  become  surrounded  by  the 
recently-formed  matrix,  which,  at  first  small  in  amount,  soon  in- 
creases so  that  the  groups  of  cartilage-cells  become  separated  by 
more  extensive  tracts  of  intercellular  substance  ;  as  the  nests  of  cells 
formed  by  the  division  of  the  original  single  occupant  of  the  lacuna 
recede  from  the  perichondrial  surface  they  lose  their  primary  parallel 
disposition  and  become  irregularly  arranged  and  further  separated. 
Sometimes  in  those  portions  most  removed  from  the  perichondrium 
the  ground-substance  appears  granular ;  this  feature  is  intensified 
when  a  deposition  of  calcareous  matter  takes  place,  which  not  infre- 
quently happens  in  old  subjects. 

Elastic  cartilage   is  distinguished  by  the  presence  of  elastic 
fibres  within  the  intercellular  substance.     The  typical  hyaline  matrix 

is  confined  to  areas  of 
Fig.  52. 

*c4\ 


Fig.  51. 


limited  extent  immedi- 
ately surrounding  the 
cell-nests,  while  the  in- 
tervening matrix  is 
penetrated  by  net- 
works of  elastic 
fibres  extending  in  all 
directions.  The  cells 
within  the  lacunae,  in 
the  midst  of  the  hy- 
aline areas,  resemble 
closely  the  usual  ele- 
ments of  hyaline  car- 
tilage. Elastic  cartilage 
has  a  much  less  general 
distribution  than  the 
hyaline  variety,  occur- 
ring principally  in  the 
cartilages  of  the  ex- 
ternal ear,  part  of  the 
Eustachian  tube,  epi- 
glottis, arytenoid  cartilages,  cartilages  of  Wrisberg  and  of  Santorini. 
This  tissue  presents  an  opaque,  yellowish  tinge  in  contrast  to  the 


Elastic  cartilage  from  the  epi- 
glottis :  c,  cartilage-cells  sur- 
rounded by  a  very  limited  area  of 
hyaline  matrix  (/;) ;  the  remaining 
part  of  the  intercellular  substance 
is  penetrated  by  net-works  of 
elastic  fibres  (e),  cross-sections  of 
■which  appear  as  minute  points. 


Fibro-cartilage  from  the 
knee-joint :  c,  cartilage- 
cells  surrounded  by  very 
limited  areas  of  hyaline 
matrix  (k)  ;  the  space  be- 
tween these  areas  is  occu- 
pied by  the  fibrous  tissue. 


THE   CONNECTIVE   TISSUES. 


47 


opalescent,  bluish  tint  of  the  hyaline  variety.  It  is  covered  by  a 
perichondrium  of  the  usual  description. 

Fibro-cartilage,  as  implied  by  its  name,  is  largely  composed  of 
interlacing  bundles  of  fibrous  connective  tissue,  embedded  in 
which  the  round  or  oval  cartilage-cells  lie,  singly  or  in  groups, 
immediately  surrounded  by  a  narrow  zone  of  hyaline  matrix. 
The  number  of  the  cells  and  the  proportion  of  fibrous  tissue  present 
differ  in  various  specimens. 

Fibro-cartilage  is  found  in  comparatively  few  localities  :  around 
the  margin  of  articular  surfaces  and  within  certain  joints,  the  sym- 
physes and  the  intervertebral  disks,  constitute  its  chief  distribution. 
The  tissue  is  closely  akin  to  tendon,  presenting  a  white,  tough,  re- 
sistant but  pliable  tissue.     A  proper  perichondrium  is  wanting. 

The  development  of  cartilage  proceeds  directly  from  the  ele- 
ments of  the  mesoderm.  The  primary  close  aggregation  of  the 
embryonal  cells,  which  early  indicates  the  position  of  the  future 
cartilage,  subsequently  gives  way  to  a  looser  disposition  of  the  cells, 
resulting  from  the  appearance  of  the  young  matrix.  After  the 
formation  of  the  perichondrium,  the  cartilage  grows  by  the  addition 
of  new  layers  beneath  the  membrane. 


Fig.  53. 


BONE. 

Bone  is  a  dense  form  of  connective  tissue  impregnated  with  lime 
salts.     Composed  of  the  same  histological  elements  as  other  compact 
connective  tissues,  bone  differs  from  these  in  having  a  deposit  of 
calcareous  matter  within  the  interfascicular 
cement-substance,  to  which  peculiarity  the 
well-known  hardness  of  the  tissue  is  due. 
The    microscopical    appearance    of   bone 
varies  with  the  character  of  the  prepara- 
tion,  especially  as  to  whether  the  earthy 
matter  has  been  removed  before  sectioning, 
or  whether  thin  plates  of  dried  bone  are 
examined ;  it  is  in  sections  of  dried  bone 
that  the  classical  pictures  of  this  tissue  are 
seen. 

Dependent  upon  the  arrangement  of  the 
matrix,  two  varieties  of  bone  are  recog- 
nized— spongy  and  compact.  Although 
the  spongy  bone  is,  as  we  shall  see,  the 
fundamental  form,  yet  the  compact  variety 
alone  presents  all  the  structural  peculiarities 
of  the  tissue.  A  transverse  section  of  the  compact  osseous  tissue 
constituting  the  shaft  of  one  of  the  long  bones  presents  a  number 


Transverse  section  of  dried  bone  : 
h,  one  of  the  Haversian  canals, 
about  which  the  lamellae  are  con- 
centrically disposed,  constituting 
the  Haversian  systems ;  g,  the 
ground  or  interstitial  lamellae. 


48 


NORMAL   HISTOLOGY. 


Fig.  54. 


Longitudinal  section  of  dried 
bone  :  h,  Haversian  canals  opened 
lengthwise  and  bordered  by  the 
longitudinally-cut  lamellae. 


of  round  or  oval  openings — the  Haversian  canals — each  sur- 
rounded by  a  broad  band  or  zone  composed  of  concentrically- 
disposed  lamellae ;  the  canal  and  the 
surrounding  lamellae  form  an  Haversian 
system.  Seen  in  longitudinal  sections, 
the  Haversian  canals  appear  as  extended 
channels,  some  closely  corresponding  in 
their  course  with  the  general  axis  of  the 
bone,  while  others  run  obliquely  and  es- 
tablish free  communication  between  the 
adjacent  canals.  The  concentric  bone 
lamellae  in  such  sections  appear  as  parallel 
bands  bordering  the  large  channels.  The 
Haversian  canals  communicate  with  the 
central  marrow-cavity,  of  which  they 
are  really  continuations ;  variable  in  width 
and  length,  each  canal  contains  an  extension 
of  the  bone-marrow,  comprising  a  delicate 
connective-tissue  reticulum,  rich  in  cells, 
blood-vessels,  and  lymphatics. 

The  areas  between  the  Haversian  systems 
are  filled  out  by  osseous  lamellae,  disposed 
without  regard  to  the  concentric  systems ;  these  are  the  interstitial 
or  ground  lamellae,  and  represent  the  older  parts  of  the  bone,  being 
the  remains  of  the  primary  spongy  net-work  of  periosteal  bone. 
The  concentric  lamellae  constituting  the  Haversian  systems  are 
secondarily  deposited  within  the  enlarged  spaces  of  the  bony  reticu- 
lum. In  addition  to  the  lamellae  already  mentioned,  superficial  os- 
seous strata  encircle  the  bone  on 
FlG-  55-  both  its  outer  and  inner  (medul- 

lary) free  surfaces ;  these  are 
the  outer  and  inner  circum- 
ferential or  fundamental  la- 
mellae. 

Between  the  bundles  of  the 
ground  -  matrix  spindle  -  shaped 
spaces — the  lacunae — are  seen, 
from  which  minute  channels — the 
canaliculi — radiate  in  all  direc- 
tions ;  these  dark,  stellate  figures  with  their  minute  lateral  canals  form 
a  system  of  intercommunicating  lymph-spaces  within  the  bone  ;  the 
canaliculi  belonging  to  the  same  space  or  to  the  adjoining  lacunae 
of  the  same  Haversian  system  anastomose  with  one  another,  but  not 
with  the  canals  of  different  systems. 


lacunae  and  canaliculi  of  dried  bone  under 
high  amplification. 


THE   CONNECTIVE   TISSUES. 


49 


Fig.  56. 


m 


A  bone-cell  lying 
within  the  lacuna  of 
the  osseous  matrix : 
decalcified  and 
stained. 


In  dried  bone  the  spaces  are  filled  with  air,  the  lacunae  and  cana- 
liculi  consequently  appearing  dark  and  sharply  defined  when  viewed 
by  transmitted  light.  The  lacunae,  sometimes  improperly  called 
"bone-cells,"  in  dried  preparations  are  empty,  or,  at  most,  contain 
the  remains  of  the  soft,  protoplasmic  bodies,  the  true  bone-cells, 
which  during  life  partially  fill  the  spaces  ;  these,  like  the  cells  of  other 
dense  connective  tissues,  lie  within  the  lymph-spaces  of  the  ground- 
matrix.  In  sections  of  young,  well-stained,  decalci- 
fied fresh  bone,  after  the  usual  manipulations,  the 
bone-corpuscles  are  seen  as  nucleated,  stellate, 
protoplasmic  bodies,  whose  processes  extend  into 
the  canaliculi ;  in  adult  and  old  bones,  however,  the 
cells  become  reduced  in  size  and  very  inconspicuous. 
The  lacunae  being  lenticular,  they  present  different 
figures  according  to  the  direction  in  which  they  are 
sectioned  :  cut  transversely,  they  appear  as  short, 
narrow  ovals  ;  opened  longitudinally,  but  not  parallel 
to  the  lamellae,  they  are  seen  as  long,  narrow,  elliptical  figures  ;  while 
when  cut  longitudinally,  and  at  the  same  time  parallel  to  the  lamellae, 
they  present  a  broad,  oval  surface,  sometimes  almost  circular ;  the 
canaliculi,  extending  in  all  planes,  appear  much  the  same  in  all 
sections. 

The  periosteum,  an  envelope  of  vascular  connective  tissue, 
closely  invests  the  outer  surface  of  all  bones  except  the  articular 
facets.  This  important 
structure  is  composed  of 
two  portions — an  outer, 
dense,  protective,  fibrous 
layer,  and  an  inner,  much 
looser  stratum,  rich  in 
cells  and  blood-vessels, 
which,  from  its  intimate 
relations  to  the  formation 
of  bone,  is  known  as 
the  osteogenetic  layer. 
This  latter  contains  within 
its  meshes  numerous 
round  or  spindle  cells, 
many  of  which  later  be- 
come bone-forming  ele- 
ments— the  osteoblasts. 

If  a  decalcified  bone  be  sectioned  parallel  to  the  superficial  lamellae, 
especially  if  these  be  of  a  spongy  bone,  or  if  the  outer  lamellae  be 
forcibly  torn  off,  a  number  of  transverse  or  perpendicular  fibres  of 

4 


Fig 


Fragments  torn  from  the  surface  of  a  decalcified  bone  :  A, 
surface  ;  B,  oblique  view  ;  s,  Sharpey's  perforating  fibres  ; 
/.  the  lacunar. 


c0  NORMAL    HISTOLOGY. 

more  or  less  delicacy  will  be  exposed ;  these  are  the  perforating 
fibres  of  Sharpey,  and  represent  periosteal  fibres  which  have  failed 
to  undergo  calcification  ;  of  these  Kolliker  recognizes  two  kinds — 
those  entirely  soft  and  uncalcified,  the  most  numerous  and,  at  the 
same  time,  the  smallest ;  and  those  partly  calcified  and  of  larger  size, 
which,  in  fact,  are  bundles  of  fibrous  tissue.  Sharpey' s  fibres  are 
most  numerous  in  the  superficial  lamellae  of  spongy  bones,  although 
found  in  the  interstitial  lamellae  of  other  bones,  pinning  together  the 
lamellae  which  they  transfix.  The  perforating  fibres,  being  derived 
from  the  periosteum,  never  occur  in  the  lamellae  of  the  Haversian 
systems,  since  the  latter,  it  will  be  found,  are  not  directly  produced 
by  the  periosteum,  but  as  secondary  deposits. 

Additional  elements  of  the  bone-matrix  are  the  elastic  fibres, 
which  are  found  in  the  outer  fundamental  lamellae,  as  well  as  occasion- 
ally in  the  deeper  interstitial  lamellae;  these  elastic  fibres  are  generally 
associated  with  the  uncalcified  Sharpey' s  fibres  ;  not  infrequently  the 
elastic  fibres  are  contained  within  the  uncalcified  bundles  of  fibrous 
tissue  composing  the  large  perforating  fibres. 

Marrow  of  Bone.  The  cavities  within  bones,  as  well  as  the 
elaborate  intercommunicating  nutrient  channels  extending  through- 
out the  osseous  tissue,  are  filled  with  the  highly  vascular  marrow, 
which  genetically  is  an  extension  of  the  osteogenetic  layer  of  the 
periosteum,  since  the  primary  marrow  is  a  direct  ingrowth  and  ex- 
tension of  this  latter  tissue.  The  marrow  of  all  bones  in  very  young 
animals  is  red  in  color ;  after  a  certain  time,  however,  that  con- 
tained within  the   shafts  of  the  tubular  and   the  spaces  of  some 

other  bones  assumes  a  lighter 
Fig.  58.  tint,    finally   becoming   of  a 

straw    color,    owing    to    the 
•  _,.-  accumulation    of   fat   within 

the   marrow-cells.     Depend- 
1  „  ing  upon  this  difference,  two 

I     varieties — the  red   marrow 
and  the  yellow  marrow — 
-        ".  are   recognized :   it  is  to  be 

remembered    that    the    red 

Elements  of  the  bone- marrow:  g,  multinucleated  marrOW  i.S  genetically  the 
giant-cell,  or  myeloplax  ;  ,„,  marrow-cells  ;  «,  granule-       ^^       ^       represents       the 

primary  condition. 
The  elements  of  the  red  marrow  comprise  a  delicate  connective- 
tissue  reticulum  supporting  a  rich  vascular  distribution,  composed  of 
arterioles  breaking  up  into  numerous  capillaries,  which,  in  turn,  give 
place  to  venous  radicles  of  large  size  and  extremely  thin  walls.     The 


THE   CONNECTIVE   TISSUES.  CT 

meshes  of  the  tissue  contain  great  numbers  of  soft,  plastic  connective- 
tissue  elements,  the  marrow-cells  ;  many  of  these,  in  actively- 
growing  bone,  become  the  osteoblasts.  In  yellow  marrow  the 
majority  of  the  marrow-cells  have  undergone  transformation  into 
fat-cells.  Additional  huge,  irregular,  multinucleated,  protoplasmic 
masses  are  occasionally  encountered;  these  are  the  giant-cells,  or 
myeloplaxes  (Robin),  and  are  of  interest  as  being  elements  es- 
pecially concerned  in  the  absorption  of  osseous  tissue,  being  iden- 
tical with  the  osteoclasts  (Kolliker).  These  cells,  with  their  nuclei, 
offer  an  example  of  what  formerly  was  described  as  the  endogenous 
mode  of  cell-formation. 

Dentine  is  analogous  to  bone,  although  differing  in  details  of 
arrangement,  since  it  is  derived  from  embryonal  connective  tissue. 
The  matrix  becomes  calcified,  and  contains,  embedded  within  the 
ground-substance,  numerous  long,  parallel,  partly-branched  tubes, 
the  dentinal  tubules.  These  correspond  with  the  lacunae  of  bone, 
enclosing  in  some  places  delicate  processes,  the  dentinal  fibres. 
A  more  extended  account  of  the  structure  and  development  of  den- 
tine will  be  found  in  connection  with  the  structure  of  the  teeth. 

Development  of  Bone.     With  the  exception  of  the  bones  of 
the  vault  of  the  cranium,  of  the  face,  and  of  part  of  the  lower  jaw, 
the  skeleton  is  mapped  out,  in  its  foetal  condition,  by  solid  cartilages 
which  correspond  in  form  more  or  less 
closely  with    the    future   bones.      The  Fig.  59. 

primary  embryonal  cartilage  is  of 
the  hyaline  variety,  being  extremely 
rich  in  cells,  many  of  which  are  engaged 
in  division;  the  cell-groups  are  separated 
by  a  relatively  small  amount  of  inter- 
cellular substance,  and  the  outer  surface 
of  these  solid  cartilages  is  closely  in- 
vested by  an  important  membrane,  the 
primary  periosteum. 

When    bone    is  formed    at  the   Centres  Primary  embryonal   cartilage   repre- 

c  . ,.  .   t  .         -  ..  .      .  senting   one  of  the   carpal   bones  :   p, 

Of    OSSlhcatlOn  Within  the   Cartilage,    It    IS       perichondrium,  or  primary  periosteum  ; 

termed  endochondral  bone  ;  when  n>  nutrient  canals  extending  from  the 
formed  directly  from  and  beneath  the  penp  ery' 
periosteum,  periosteal  bone.  While  quite  complicated  in  its 
sequence  of  changes,  it  must  be  remembered  that  endochondral 
development  results  in  the  formation  of  structures  which  are  largely 
temporary,  and  which  finally,  for  the  most  part,  suffer  absorption. 
The  permanent  bones  of  the  skeleton  are,  chiefly,  the  products  of 


r2  NORMAL   HISTOLOGY. 

the  periosteum  ;  where  bone  is  developed  directly  from  the  periosteum, 
and  without  being  mapped  out  by  primary  cartilage,  the  process  is 
spoken  of  as  intermembranous  bone-formation,  although  differ- 
ing in  no  important  respect  from  that  producing  the  periosteal  bone. 

Endochondral  Bone.  The  first  indications  of  the  future  pro- 
found changes  within  the  solid  cartilage  correspond  in  position  to 

the  so-called  centres  of  ossification, 

Fig.  60.  and  consist  in  an  increase  in  the  size 

»    %»  o  i>%  &%  §®®  ©  y  °f  ^e  embryonal  cartilage-cells,  as  well 

®?«®f<i^'A??«^-tli^{*^I  as  m  tne  amount  of  intercellular  sub- 

«  %•  -  «  ""','.  '  ~  \  stance  separating  the  cell-nests,  followed 

A^f      by  a  characteristic  rearrangement  of 

V'.V  .-:  A      the  enlarged  cells  into  vertical  rows 

\  '■,'.'/.  "''•  !     or  columns  ;  in  the  matrix  between  and 

j-     Y-V.*' /;* ■.   :  '.'"i     around   these  columnar  groups  a  cal- 

'V    \  '..    ..  ~-  /--,'/     careous    deposit    subsequently    takes 

%  j^fjfl^fv f   ",  ".  ,' '.}':  "',*'*/       place.     These  enlarged  cartilage-cells, 

sS"c "  "  ^ -•"    ®*  "  surrounded  by  the  calcified  matrix,  are 

.iiSSSi  wTra"a5e°r;ad     the  primary  areola  of  Sharpey. 

cartilage- lacunae  and  calcified  matrix;  c,  Simultaneously      with      the       changes 

young  cartilage-cells.  noted     the     osteogenetic     tissue    of    the 

periosteum  has  increased  and  sent  processes  from  a  number  of  points 
into  the  solid  cartilage  towards  the  centre  of  ossification  ;  the  progress 
of  the  periosteal  ingrowth  is  accompanied  by  the  absorption  of  the 
cartilage  until  the  focus  of  central  calcification  is  reached,  when  the 
greatly  enlarged  cartilage-lacunas  are  opened  up  and  the  spaces 
brought  into  direct  communication  with  the  primary  marrow- 
cavities.  The  fate  of  the  cartilage-corpuscles  has  been  the  sub- 
ject of  discussion ;  it  may  be  assumed  as  established  that  these  cells 
undergo  degeneration  and  play  no  part  in  the  formation  of  the  new 
bone.  This  periosteal  ingrowth  constitutes  the  vascularization 
of  the  cartilage.  The  process  of  breaking  down  the  cartilage-cells 
and  opening  up  the  large  lacunas  goes  rapidly  forward,  resulting  in 
the  extension  of  the  primary  marrow-cavity;  the  primary  marrow, 
filling  this  latter  space,  is,  as  already  pointed  out,  the  direct  deriva- 
tive of  the  inner  layer  of  the  periosteum. 

The  primary  marrow-cavity,  or  medullary  space,  soon  becoming 
of  considerable  size,  is  bordered  by  the  zone  of  calcifying  cartilage  ; 
this  area  includes  the  columns  of  flattened  cells  and  enlarged  lacunae, 
which  pass  into  the  broken  and  partly-absorbed  larger  lacunae,  the 
secondary  areolae,  opening  into  the  primary  marrow-cavity. 

While  the  horizontal  matrix  septa  between  the  transversely  ex- 
panded lacunae  disappear,  the  vertical  partitions  lying  between  the 


THE    CONNECTIVE   TISSUES. 


53 


columns  of  the  cells  suffer  much  less  reduction,  and,  as  a  result, 
remain  and  project  into  the  marrow-cavity  as  irregular  trabeculae  of 
calcified  cartilage.  The  marrow-cells  rapidly  multiply  and  arrange 
themselves  as  a  layer  upon 

the  surface  of  the  cartilage-  FlG-  6l- 

trabeculae  ;      now     called 
osteoblasts,    they    busy 
themselves  in  enveloping 
these  with  a  covering  of 
true  osseous  tissue.     Si- 
multaneously    with     the 
deposition  of  the  bone  the 
calcified   cartilage   within 
the  trabeculae  undergoes 
absorption,    so    that    the 
amount   of  cartilage   en- 
cased   by  the    new  bone 
gradually  diminishes  and    j 
finally      disappears,      the   U 
entire  net-work  of  anas-    ■ 
tomosing  trabeculae  being    [ 
now  composed  of  true  os-    -j 
seous  tissue.    This  newly-    \ 
formed   net-work   consti-    ;. 
tutes     the     central    [ 
primary  spongy  bone, 
a  structure  which,  in  the 
shafts  of  the  long  bones, 
is   but   temporary,    after- 
wards entirely  disappear- 
ing, except  at  the  ends  of 
the  bones,  where  it  per- 
sists   as    the     cancellous 
tissue  of  the  extremities. 

It  will  be  noticed  that 
in  the  changes  above  described  the  cartilage  is  not  directly  converted 
into  bone,  ossification  being  a  process  of  substitution,  the  new  bone 
replacing  the  primary  cartilage. 

Starting  near  the  middle  of  the  long  bones,  the  process  of  calci- 
fication and  absorption  of  the  cartilage  and  the  formation  of  the 
primary  spongy  bone  proceed  towards  the  extremities,  the  original 
cartilage  gradually  disappearing,  the  loss  being  made  up  by  incre- 
ments of  new  cartilage  deposited  on  the  surface  beneath  the  peri- 
chondrium. 


Developing  bone — from  the  end  of  a  long  bone  :  a,  area 
of  rearranging  cartilage-cells  ;  e,  area  of  enlarged  lacunae ;  c, 
zone  of  calcined  matrix  ;  tn,  primary  marrow-spaces  contain- 
ing the  osteogenetic  tissue  ;  b,  trabecular  of  new  bone  cover- 
ing the  remains  (r)  of  the  calcified  cartilage-matrix. 


ca  NORMAL   HISTOLOGY. 

Periosteal  Bone.  Simultaneously  with  the  formation  of  the 
central  spongy  endochondral  bone  the  cells  of  the  osteogenetic  layer 
of  the  periosteum  are  actively  engaged  in  likewise  producing  osseous 

tissue,  the  trabecular  of  which  unite 
to  form  the  peripheral  net-work  of 
periosteal    bone,   this   in    many 

Fig.  63. 


Developing  bone — trabecula  of  endochondral 
bone  :  a,  the  new  bone ;  6,  bone-cells ;  c,  still 
unabsorbed  remains  of  calcified  cartilage-matrix. 


Developing  bone — the  surface  of  portion  of 
bone-trabecula,  exhibiting  the  conversion  of  the 
osteoblasts  into  the  bone-corpuscles  :  b,  lacuna 
with  young  bone-cell ;  o,  osteoblasts  arranged  on 
the  surface  of  the  newly-formed  osseous  matrix 
(m) ;  at  /  an  osteoblast  just  being  isolated. 


places  forming  an  outer  envelope  closely  embracing  the  central  endo- 
chondral bone. 

The  details  of  the  process  by  which  the  osteoblasts  are  converted 
into  the  bone-cells  are  the  same  in  both  the  intracartilaginous  and 
the  periosteal  formation.  The  bone-matrix,  deposited  through  the 
agency  of  the  cells,  gradually  accumulates  around  the  osteoblast,  until 
this  lies  completely  surrounded  by  the  young  matrix,  when,  after  its 
isolation  from  the  marrow-cavity,  it  becomes  the  bone-corpuscle. 
At  first  the  canaliculi  are  wanting,  as  are,  also,  calcareous  matters ; 
these  later  appear. 

The  conversion  of  the  original  spongy  into  compact  bone 
depends  upon  the  development  of  additional  lamellae  within  the 
meshes  of  the  primary  osseous  net-work.  As  an  initial  step,  a  local 
absorption  takes  place,  resulting  in  the  enlargement  of  the  pri- 
mary medullary  spaces  contained  between  the  trabecular  of  the 
periosteal  net-work;  these  osseous  bands  are  thus  reduced  to  thin 
bony  partitions  between  large  oval  cavities,  the  Haversian  spaces. 
A  new  growth  of  bone  subsequently  takes  place  within  these  spaces, 


THE   CONNECTIVE   TISSUES.  re 

the  osteoblasts  depositing  new  bone  upon  the  walls  of  these  cylin- 
drical cavities,  layer  upon  layer,  until  only  a  small  central  channel — 
the  Haversian  canal — remains  as  the  representative  of  the  large 
Haversian  space.  The  outer  boundary  of  the  Haversian  system, 
therefore,  corresponds  to  the  limits  of  the  Haversian  space,  while 
the  remains  of  the  primary  bone-trabeculae  constitute  the  older 
interstitial  lamellae  of  the  adult  tissue. 

Osseous  tissue,  wherever  developed,  is  formed  through  the 
agency  of  the  osteoblasts,  the  deriva- 
tives and  descendants  of  the  special- 
ized mesoblastic  cells  of  the  embryo ; 
whether  in  endochondral  or  periosteal 
formation,  the  bone-producing  elements 


Fig.  65. 


Developing  bone — both  periosteal  and  endochondral : 
f,  outer  fibrous,  o,  inner  osteogenetic  layer  of  perios- 
teum ;  /,  trabecular  of  periosteal  bone  covered  by  the 
osteoblasts  ;  e,  endochondral  bone  ;  m,  primary  marrow- 
cavities. 


Developing  bone — longitudinal  section 
of  embryonal  phalanx  :  e,  the  primary 
cartilage  of  the  extremities  of  the  bone ; 
a,  zone  of  enlarged  and  vertically-dis- 
posed cartilage-lacunae  ;  c,  zone  of  calcifi- 
cation ;  t,  trabeculse  of  calcified  cartilage 
covered  with  new  bone ;  ?n,  marrow- 
cavity  ;  b,  periosteal  bone  formed  directly 
beneath  the  overlying  periosteum,/. 


arrange  themselves  over  the  surfaces  of  the  cartilage-trabeculae  or  the 
periosteal  fibres  respectively,  and  soon  are  surrounded  by  osseous 


Developing  bone— portion  of 
trabecula  undergoing  absorp- 
tion :  b,  bone-cells ;  c,  osteo- 
blasts ;  m,  bone-matrix  ;  o,  multi- 
nucleated osteoclast  lying  within 
the  absorption-pit,  or  Howship's 
lacuna. 


c6  NORMAL   HISTOLOGY. 

matrix  ;  this  gradually  thickens  and  encloses  the  osteoblasts,  which 
now  lie  within  minute  bays  or  recesses,  the  entrances  to  which  become 
gradually  contracted,  until  the  opposed 
edges  join  and  the  cells  lie  within  lacunae 
completely  surrounded  by  the  bone-matrix  : 
the  osteoblasts  have  now  become  the  bone- 
cells.  The  matrix  is  deposited  as  lamellae, 
especially  marked  in  the  bone  formed  in  the 
later  stages  of  fcetal  life  ;  between  these  are 
included  the  lacunae.  The  matrix  is  at  first 
soft  and  possessed  of  a  distinct  fibrillated 
structure  in  which  the  subsequent  deposit 
of  lime  salts — principally  the  phosphate  and 
carbonate — takes  place. 

When,   on  the   contrary,   bone   or  carti- 
lage is  absorbed,  it  is  through  the  agency 
of  the  giant-cells,  the  osteoclasts,  or  chon- 
droclasts  (Klein);  these  large  multinucleated 
elements  usually  lie  upon  the  surface  of  the  bone-trabeculae  within 
larger  or  smaller  pits  which  have  been  excavated  by  them  ;  these 
are  Howship's  lacunae. 

In  recapitulation,  the  following  summary  of  the  phases  of  de- 
velopment during  the  growth  of  a  tubular  long  bone  may  be 
noted  : 

i.   Solid  embryo7ial  cartilage. 

2.  Enlargement  and  rearrangement  of  cartilage-cells  and  lacunae 
and  calcification  of  matrix  at  centre  of  ossification. 

3.  Penetration  of  periosteal  tissue  to  the  focus  of  calcification  ; 
vascularization  of  the  cartilage. 

4.  Formation  of  medullary  spaces  by  the  breaking  down  of  lacunae 
surrounded  by  the  zone  of  calcifying  cartilage. 

5.  Coveri?ig  of  the  surface  of  calcified  cartilage  trabeculae  by  the 
layer  of  osteoblasts  and  the  productio?i  of  a7i  enveloping  sheath  of 
true  bone. 

6.  Resulting  central  net-work  of  endochondral  bone,  with  gradual 
absorption  of  encased  cartilage  trabeculae. 

7.  Absorption  of  central  spo?igy  bone  in  shaft  and  formation  of 
central  marrow-cavity. 

8.  Formation,  meanwhile,  of  peripheral  periosteal  net-work  of 
spongy  bone. 

9.  Conversion  into  compact  bone  by  partial  absorption  of  tra- 
beculae to  form  Haversian  spaces ;  secondary  deposit  of  concentric 
lamellae  within  these  spaces  forming  Haversian  systems  of  compact 
bone. 


THE   CONNECTIVE   TISSUES.  ry 

10.  Absorption  of  inner  lamellcz  of  compact  bone  as  the  shaft 
increases  in  diameter  by  the  deposition  beneath  the  periosteum ; 
production  of  enlarged  medullary  cavity. 

n.  Continued  absorption  of  endocho?idral  central  bone  until  the 
latter  is  found  alone  in  the  epiphyses,  where  it  continues  to  be  pro- 
duced at  the  expense  of  the  intermediate  cartilage  during  the  entire 
future  growth  of  the  bone. 


58 


NORMAL   HISTOLOGY. 


CHAPTER    IV. 

THE    MUSCULAR   TISSUES. 

Contractility  is  possessed,  to  a  certain  degree,  in  common  by- 
all  cells  rich  in  active  protoplasm  ;  the  distinguishing  characteristic 
of  muscular  tissue,  however,  is  that  this  property  is  so  conspicuously 
developed  in  highly  specialized  structures,  and  that  the  contractions 
take  place  along  definite  lines  in  limited  directions  alone.  Con- 
tractile tissue  or  muscle  occurs  in  two  principal  forms:  (i)  as  the 
non-striated,  smooth,  or  vegetative  muscle,  usually  beyond 
the  control  of  the  will,  and  hence  called  involuntary \  and  (2)  as  the 
striated,  striped,  or  animal  muscle,  which,  being  influenced  by 
volition,  is  known  as  voluntary. 

The  sharp  differences  separating  the  two  groups  of  muscle  in  man 
and  the  higher  animals  cannot  be  regarded  as  fundamental,  since  in 
the  embryonal  condition  of  these  higher  forms  temporarily,  and  in 
the  adult  form  of  the  lower  types  permanently,  the  striped  and  non- 
striated  varieties  of  muscle  depend  upon  the  degree  of  specialization 
rather  than  upon  inherent  differences.  It  is  a  suggestive  fact  that 
long  before  the  cells  forming  the  embryonal  heart  show  indications 
of  differentiation  into  muscle-tissue  the  contractions  of  the  organ 
have  commenced.  The  association  of  the  striped  fibres  with  response 
to  the  will  and,  on  the  contrary,  of  the  plain  tissue  with  involuntary 
action  must  be,  likewise,  only  provisionally  accepted,  since  in  some 
animals  the  development  of  marked  striae  never  takes  place  in  the 
voluntary  fibres.  Standing  between  and  connecting  the  extremes 
of  these  groups  is  the  cardiac  muscle  of  the  higher  vertebrates,  in 
which  the  fibres  are  striated,  although  beyond  the  control  of  the  will. 

NON-STRIATED    OR    INVOLUNTARY    MUSCLE. 

Non-striated,  smooth,  or  involuntary  muscle,  while  never  occurring 
in  large  individual  masses,  enjoys  a  wide  distribution  ;  its  principal 
localities  are — 

1.  The  Digestive  Tract:  the  muscularis  mucosae  from  oesophagus 
to  anus  and  the  delicate  bundles  of  mucosa  and  villi ;  muscular  tunic 
from  the  lower  half  of  oesophagus  to  anus. 

2.  The  Accessory  Digestive  Glands :  in  the  large  excretory  ducts 
of  liver,  pancreas,  and  some  salivary  glands  ;  also  in  the  gall-bladder. 

3.  The  Urinary  Tract :  in  the  capsule  and  the  pelvis  of  kidney, 
ureter,  bladder,  and  urethra. 


THE   MUSCULAR   TISSUES. 


59 


4.  The  Male  Generative  Organs :  in  epididymis,  vas  deferens, 
vesiculae  seminales,  prostate  body,  Cowper's  glands,  cavernous  and 
spongy  bodies  of  penis. 

5.  The  Female  Generative  Orga?is :  in  oviducts,  uterus,  and 
vagina  ;  in  the  erectile  tissue  of  external  genitals  ;  in  broad  and  round 
ligaments,  and  in  erectile  tissue  of  nipple. 

6.  The  Respiratory  Tract :  in  the  posterior  part  of  trachea ; 
encircling  bands  in  bronchial  tubes,  and  bundles  within  pleura. 

7.  The  Vascular  System :  in  the  coats  of  arteries,  veins,  and 
larger  lymphatics. 

8.  The  Lymphatic  Glands :  in  the  capsule  and  the  trabeculae  of 
spleen  ;  sometimes  in  the  trabecular  of  lymphatic  glands. 

9.  The  Eye  :  in  iris  and  ciliary  body,  and  in  eyelids. 

10.  The  Integument:  as  the  arrectores  pili  connected  with  the 
hair-follicles  ;  in  sweat  and  some  sebaceous  glands  ;  in  skin  covering 
the  scrotum  and  parts  of  the  external  genitals. 

Involuntary  muscle  is  composed  of  delicate  spindle,  often  rib- 
bon-like, fibre-cells  ;   these  vary 
greatly  in  size,  measuring  75-225  Fig. 

pt*  long  and  4-8  ft  wide.      The 
cells   found   in  arteries   are  short 

Fig.  67. 


Isolated  involuntary-muscle  cells  from   intestine 
of  man. 


Involuntary-muscle  cells  from  mesentery  of 
newt :  n,  nuclei ;  /,  axial  fibre ;  m,  transverse 
markings  on  surface  of  cell ;  B,  muscle-cell  with 
forked  extremity. 


and  flat,  being  but  25-45  P  ^onS  an<^  9-12  ft  wide;  the  largest  ele- 
ments are  found  in  the  gravid  uterus,  where  they  reach  a  length 


*  1  il  (microti)  =  the  1000th  part  of  a  millimetre. 


6o 


NORMAL   HISTOLOGY. 


Fig.  69. 


of  over  500  /-t  and  a  breadth  of  20  ft.  Occasional  cells  with  bi- 
furcated ends  are  encountered,  especially  among  the  lower  verte- 
brates. 

The  spindle  muscle-cell  is  invested  with  a  very  delicate,  homo- 
geneous, hyaline  sheath,  closely  resembling  elastic  tissue,  and 
corresponding  to  the  sarcolemma  of  the  striated  fibre ;  within  this 
envelope  lies  the  soft,  semi-fluid,  contractile  protoplasm,  embedded 
in  which,  near  the  centre  of  the  cell,  lies  a  characteristic,  narrow, 
rod-shaped  nucleus.  Delicate  longitudinal  fibrillae  sometimes  can 
be  made  out  extending  the  entire  length  of  the  cell ;  these  are  re- 
garded by  many  histologists  as  representing  the  actively  contractile 
parts  of  the  cell,  the  surrounding  protoplasm  being  largely  passive. 
Transverse  markings  are  also  often  seen  ;  these  correspond  in  posi- 
tion to  local  variations  in  the  diameter 
of  the  cell,  and  are  probably  due  to 
corrugations  in  the  enveloping  mem- 
brane. 

The  individual  spindle-cells  are  closely 
fitted  together  and  united  by  an  albu- 
minous cement-substance  ;  they  are  dis- 
posed  in  groups  or  bundles,  which,  on 
cross-section,  are  made  up  of  rounded 
polygonal    areas    of   varying    size,    the 
larger  possessing  round  nuclei,  while  the 
smaller  have   none.     Since   these   areas 
are   the   sections   of   nucleated   spindle- 
cells,   the   large  nucleated   fields   corre- 
spond  to  sections  passing   through  the 
nucleus  of  the  cell,  while  the  small  ones  are  sections  of  the  cell  fell- 
ing near  the  pointed  ends.     The  bundles  of  muscle-cells  are  arranged 
to  form  layers  or  sheets,  as  in  the  digestive  tract,   or  net-works, 

as    in    the    eye,    pleura,    etc. 
FlG-  7°-  Examined  in  longitudinal  sec- 

tion, or  in  considerable  masses, 
it  is  difficult  to  distinguish  the 
individual     component     fibre- 
cells,    the   involuntary   muscle 
in  such  cases  closely  resembling 
fibrous  connective  tissue  ;  how- 
ever,  the   numerous   more   or 
less    regularly    disposed    rod- 
shaped  nuclei,  and  the  absence  of  the  delicate  wavy  fibres,  together 
with  the  impression  of  greater  density,  usually  suffice  to  establish 
the  identity  of  the  muscle. 


Involuntary  muscle  in  transverse 
section  :  portions  of  three  bundles  are 
represented,  separated  by  areolar 
tissue  {a) :  the  nucleated  areas  are 
sections  of  the  muscle  cells  through 
their  nuclei ;  the  smaller  figures  repre- 
sent sections  of  the  cells  cut  nearer  the 
ends. 


Involuntary  muscle  in  longitudinal  section  :  the 
muscle-cells  are  often  cut  obliquely,  and  hence  appear 
shorter  than  when  isolated. 


THE   MUSCULAR   TISSUES. 


61 


The  connective  tissue  uniting  the  larger  bundles  of  muscle-cells 
supports  the  blood-vessels  and  nerves.  The  larger  blood-vessels 
break  up  into  capillary  net-works,  which  pass  between  the  muscle- 
cells.  The  nerves,  derived  principally  from  the  sympathetic  system, 
likewise  penetrate  the  intercellular  spaces  and  terminate  between  the 
cells  in  the  manner  more  fully  described  in  the  chapter  devoted  to 
nerve-endings.  Lymphatics  occur,  as  in  parts  of  the  digestive 
tract,  closely  associated  with  the  muscular  tissue. 


STRIATED    OR   VOLUNTARY   MUSCLE. 

Striated  or  voluntary  muscle,  in  addition  to  the  extensive  system 
attached  to  the  skeleton,  supplies  the  special  muscles  connected  with 
many  organs,  including  the  tongue,  pharynx,  middle  ear,  larynx, 
upper  half  of  the  oesophagus,  diaphragm,  generative  organs,  etc. 
This  form  of  muscle  is  composed  of  long,  irregularly  cylindrical  fibres, 
each  of  which  represents  the  high  specialization  resulting  from  the 
development  of  the  single  original  embryonal  cell ;  the  fibre  is, 
therefore,  the  structural  unit  of  the  striated  muscular  tissue,  and 
corresponds  to  the  spindle  fibre-cell  of  the  involuntary  variety.  The 
fibre  of  striped  muscle  comprises  (a)  the  sarcolemma,  (b)  the  muscle- 
nuclei,  and  (c)  the  muscle-substance. 

Each  fibre,  is  closely  invested  by  a  clear,   homogeneous,   elastic 
sheath — the  sarcolemma — which,  ordinarily,  so  tightly  adheres  to 
the  enclosed  muscle-substance  that  the  two  are 
optically  blended  together;  in  favorable  positions,  Fig.  71. 

as  where  breaks  in  the  sarcous  substance  occur, 
or  after  the  action  of  water,  the  sarcolemma  is 
separated  from  the  muscle-substance,  and  is  then 
seen  in  profile  as  a  delicate  line  spanning  the 
break  in  the  continuity  of  the  fibre.  The  sar- 
colemma forms  a  closed  sac  completely  envel- 
oping the  contractile  substance  of  the  fibre. 

Immediately  beneath  the  sarcolemma,  lying 
within  minute  depressions  on  the  surface  of  the 
muscle-substance,  are  the  muscle-nuclei. 
These  are  oval  or  fusiform,  usually  placed 
parallel  to  the  long  axis  of  the  fibre,  and  sur- 
rounded, especially  at  their  ends,  by  a  small 
amount  of  granular  protoplasm.  These  accumu- 
lations represent  the  meagre  remains  of  the 
indifferent  protoplasm  which  has  not  undergone 
conversion  into  the  highly  specialized  muscle-substance  of  the  fibre. 
In  mammalian  muscle  the  nuclei  lie  always  upon  the  surface  of  the 
sarcous  substance  of  the  fibre  and  immediately  beneath  the  sarco- 


Voluntary-muscle  fibres, 
somewhat  broken  after 
treatment  with  water, 
showing  the  sarcolemma 
(s)  in  several  places. 


62 


NORMAL    HISTOLOGY. 


lemma ;  in  the  majority  of  other  vertebrates,  however,  the  nuclei 
are  distributed  irregularly  throughout  all  parts  of  the  contractile 
substance.  These  differences  are  well  shown  in  the  accompanying 
figures. 

The  muscle -fibres  present  alternate  light  and  dark  transverse 
markings,  or  striae,  to  which  the  tissue  owes  its  characteristic  appear- 
ance. The  highly  specialized 
contents  of  the  sarcolemma  are 
composed  of  two  substances  pos- 
sessing different  refractive  prop- 
erties, that  forming  the  dark 
bands  being  doubly  refracting, 
or    anisotropic,     while     that     of 


Fig.  72. 


A 


B 


Fibres  of  voluntary  muscle  in  section :  A , 
human  fibres,  with  nuclei  upon  the  surface  and 
beneath  the  sarcolemma  ;  B,  fibres  from  frog,  with 
nuclei  embedded  within  the  muscle-substance. 


Voluntary  muscle,  portions  of  two  fibres  show- 
ing the  characteristic  transverse  markings ;  the 
lighter  band  is  divided  by  the  row  of  minute  beads 
constituting  the  intermediate  disk  :  a,  termination 
of  muscu'ar  substance  and  attachment  of  adjoin- 
ing fibrous  tissue;  n,  nuclei  of  muscle-fibres. 


the  light  striae  is  singly  refracting,  or  isotropic.  When  fresh  or 
well-preserved  mammalian  muscle  is  examined  under  high  am- 
plification it  is  seen  that  the  dark  striae,  or  transverse  disks,  are 
not  unbroken  homogeneous  bands,  but  that  each  is  composed  of  a 
number  of  minute  prismatic  elements  placed  side  by  side  and  sep- 
arated from  one  another  by  a  thin  layer  of  a  substance  corresponding 
to  and  continuous  with  that  forming  the  light  zone.  This  latter,  in 
addition,  is  divided  transversely  by  a  delicate  interrupted  line  or* 
row  of  dark  dots — the  intermediate  disk,  or  membrane  of  Krause. 
That  part  of  the  light  zone  between  the  dim  intermediate  and  trans- 
verse disks  constitutes  the  lateral  disk. 

The  explanation  of  these  appearances  has  caused  many  and  pro- 
longed discussions,  and  even  at  present,  notwithstanding  the  careful 
study  bestowed  upon  the  subject,  the  exact  structure  of  voluntary 
muscle  must  be  regarded  as  still  unsettled.  Heretofore  two  promi- 
nent and  opposed  views  have  prevailed  :  the  one  regards  the  fibre  as 
composed  of  parallel  longitudinal  rows  of  minute  prisms  forming 
fibrillae  (as  rows  of  bricks  placed  end  to  end);  the  other  considers 
the  fibre  as  built  up  by  the  apposition  of  their  disks,  whose  diameter 
corresponds  to  that  of  the  entire  fibre  (as  cheese-boxes  piled  one 


THE   MUSCULAR   TISSUES.  63 

upon  the  other).  After  treatment  with  alcohol,  the  fibres  of  striped 
muscle  readily  split  up  lengthwise  into  delicate  bundles,  which,  with 
care,  may  be  subdivided  to  such  an  extent  that  the  resulting  threads 
embrace  in  their  width  only  a  single  row  of  alternating  light  and 
dark  elements.  These  ultimate  fibrillae  were  formerly  considered 
by  Kolliker  as  the  normal  elements  of  the  fibre ;  the  dark  prisms  of 
these  fibrillae  correspond  to  the  sarcous  elements,  which  were 
regarded  by  Bowman  as  the  component  units  of  muscular  tissue. 
The  transverse  cleavage  of  the  fibre  following  the  action  of  diluted 
mineral  acids,  on  the  other  hand,  has  been  upheld  as  representing 
the  natural  division.  According  to  Krause,  the  fibre  is  divided 
through  the  light  bands  by  a  number  of  transverse  partitions  con- 
tinuous with  the  sarcolemma ;  these  assumed  septa  appear  as  delicate 
broken  lines — the  membranes  of  Krause — and  are  identical  with 
the  intermediate  disks  already  mentioned.  Adopting  this  view,  the 
fibre  is  composed  of  numerous  thin  zones  or  contractile  disks, 
each  of  which  embraces  the  dark  dim  band  in  its  centre  and  half  of 
the  light  stripe  at  either  end.  Each  contractile  disk  is  further  sub- 
divided by  vertical  partitions  extending  between  the  neighboring 
membranes  of  Krause,  thus  forming  in  every  disk  a  row  of  com- 
partments or  muscle-caskets.  The  portion  of  the  dim  band  con- 
tained within  each  muscle-casket  has  been  regarded  as  itself  being 
composed  of  a  series  of  thin  prisms  of  contractile  substance — the 
muscle-rods. 

After  renewed  critical  study  of  the  subject,  Rollett  has  presented 
a  view  regarding  the  structure  of  voluntary  muscle  which  not  only 
offers  the  most  plausible  solution  of  this  difficult  problem,  but  is, 
likewise,  in  harmony  with  the  history  of  the  development  of  the 
tissue.  According  to  this  theory,  the  muscular  tissue  is  composed 
of  the  highly  specialized,  darker,  anisotropic  contractile  substance, 
and  the  relatively  passive,  lighter,  semi-fluid,  isotropic  sarcoplasm. 
The  contractile  substance  is  arranged  as  delicate  spindles,  the  appo- 
sition of  whose  thicker  parts  produces  the  dim  transverse  disk  seen 
under  medium  amplification ;  at  either  end  the  spindle  is  prolonged 
as  an  extremely  thin  thread,  which  terminates  in  a  minute  sphere  or 
bead ;  the  apposition  of  these  beads  in  the  transverse  row  gives  rise 
to  the  appearance  of  the  interrupted  line  constituting  the  inter- 
mediate disk,  or  Krause 's  membrane.  The  darker  anisotropic  sub- 
stance forms,  therefore,  numbers  of  continuous  contractile  fibrillae, 
which  extend  in  parallel  bundles  the  entire  length  of  the  fibre ;  all 
the  remaining  interfibrillar  space  within  the  sarcolemma  is  filled  with 
the  lighter  sarcoplasm,  which  appears  faintly  granular  in  preserved 
tissue,  but  is,  probably,  almost  fluid  during  life.  On  comparing  this 
description  with  the  usual  appearances  presented  by  striated  muscle, 


Fig 


64  NORMAL    HISTOLOGY. 

it  will  be  seen  that  the  lateral  apposition  of  the  thicker  parts  of  the 
contractile  fibrillce  produces  the  dark  band,  or  transverse  disk,  while 
the  row  of  minute  spherical  masses  appears  as  the  interrupted  dark 
line  bisecting  the  light  zone,  or  intermediate  disk.  The  threads 
bridging  between  these  beads  and  the  chief 
mass  of  the  fibrillae  are  too  delicate  to  be 
appreciated  under  ordinary  powers,  and  that 
portion  of  the  fibre  corresponding  to  the 
lateral  disk  consequently  appears  as  if  made 
up  of  the  lighter  sarcoplasm  alone. 

In  certain  forms  of  invertebrate  muscle  a 
more  complicated  arrangement  exists,  since 
on  either  side  of  the  intermediate  disk  a  row 
of  dark  granules  crosses  the  light  lateral  disk, 
forming  a  dim  secondary  disk  ;  these  gran- 
ules are  connected  with  the  intermediate  and 
transverse  disks  by  delicate  bridges  of  con- 
tractile substance,  along  which  they  occur  as 
local  thickenings.  The  dim  transverse  disk 
sometimes  contains  a  central  lighter  band, 
the  median  disk  of  Hensen,  which  is  due, 
probably,  to  diminished  thickness  of  the  con- 
tractile fibrils. 

The  contractile  fibrillae,  however,  are  not 
uniformly  distributed  throughout  the  fibre, 
but  are  aggregated  into  bundles — the  muscle- 
columns — each  of  which  is  enveloped  in  a  thicker  layer  of  the 
sarcoplasm  than  the  partitions  separating  the  individual  fibrillae. 

In  transverse  section  each  muscle-fibre  presents  a  number  of 
small,  polygonal,  dark  areas,  enclosed  by  lighter  lines,  which  areas, 
under  high  amplification,  exhibit  minute  punctations.  These  areas 
are  sections  of  the  muscle-columns  and  correspond  to  Cohnheim's 
fields,  the  dots  being  sections  of  the  individual  constituent  fibrillae; 
the  lighter  intervening  and  surrounding  substance  is  the  sarcoplasm, 
thicker  layers  of  which  surround  and  separate  the  larger  groups  into 
which  the  muscle-columns  are  further  collected. 

The  individual  muscle-fibres,  which  usually  are  not  circular  in 
cross-section,  but  rather  irregularly  polygonal  with  rounded  angles, 
are  held  together  by  a  small  amount  of  areolar  tissue,  the  endo- 
mysium.  They  are  grouped  into  primary  bundles,  which  latter 
are  enveloped  and  separated  from  other  primary  bundles  by  the 
thicker  bands  of  connective  tissue  constituting  the  perimysium. 
The  primary  bundles  are  united  to  form  larger  secondary  groups  or 
fasciculi,  upon  the  width  and  arrangement  of  which  the  coarseness 


A,  diagram  of  arrangement 
of  the  contractile  substance 
according  to  the  view  of  Rol- 
lett  :  the  granular  figures  rep- 
resent the  contractile  elements, 
the  intervening  light  areas  the 
sarcoplasm  ;  B,  small  muscle- 
fibre  of  man  ;  the  correspond- 
ing parts  in  the  two  figures  are 
indicated  :  t,  i,  I,  respectively 
the  transverse,  intermediate, 
and  lateral  disks ;  n,  muscle- 
nuclei. 


THE   MUSCULAR   TISSUES.  gc 

or  the  fineness,  macroscopically  appreciable,  of  the  muscle  largely 
depends.  The  entire  muscle  is  invested  in  a  fibrous  sheath,  the 
epimysium,  derived  from  the  denser  layers  of  the  interfascicular 
connective  tissue. 

When  contraction  takes  place,  the  entire  muscle  becomes  shorter 
and,  at  the  same  time,  broader ;  the  striae  also 
participate  in  the  changes,  becoming  narrower.  Fig.  75. 

These    phenomena,    however,    affect    only   a  A  B 

limited  part  of  the  fibre  at  one  time,  consecu- 
tive portions  being  influenced  in  regular  se- 
quence, so  that  the  changes  pass  along  the 
fibre  as  a  contraction  wave ;  after  the  passage 
of  the  wave  the  muscle  resumes  its  previous 
condition. 

In  short  muscles  the  individual  fibres  quite 
frequently  extend  the  entire  length  ;  in  long 
ones,  on  the  contrary,  the  fibres  are  shorter 
than  the  muscle,  being  generally  some  30-45 
mm.  long;  sometimes,  however,  the  fibres 
reach  a  length  of  120  mm.  by  10-50  mm.  in 
width  (Felix).  The  fibres,  as  a  whole,  are  gen- 
erally somewhat  spindle-shaped,  being  slightly  larger  in  the  middle ; 
the  ends  of  the  fibres  are  more  or  less  pointed,  although  blunted 

or  club-shaped,  and,  more  rarely, 
Fig.  76.  branched,  extremities  are  not  un- 

common. Branched  and  anas- 
tomosing fibres  frequently  occur 
(Gage),  especially  in  the  tongue 

Fig.  77. 


Muscle-fibres  in  transverse 
section,  highly  magnified  :  A, 
portion  of  human  muscle  :  the 
small,  irregular  areas  are  the 
fields  of  Cohnheim  (c) ;  B, 
semi-diagrammatic  view  show- 
ing the  groups  of  muscle-col- 
umns composing  Cohnheim's 
fields  ;  n,  nucleus ;  m,  groups 
of  muscle-columns. 


Voluntary  muscle  in  transverse  section  :  the 
irregular  polyhedral  areas  (_/")  are  the  individual 
muscle-fibres  in  section,  held  together  by  the  en- 
domysium  (<?)  ;  the  primary  bundles  of  the  fibres 
are  enclosed  by  the  denser  perimysium  (fi). 


Branched  voluntary-muscle  fibres  from  the 
tongue. 


and  ocular  muscles.     When  the  individual  fibres  do  not  extend  the 
length  of  the  entire  muscle,  the  sarcous  substance  terminates   in 

5 


66 


NORMAL   HISTOLOGY. 


Fig.  78. 


pointed  or  rounded  extremities,  while  the  sarcolemma  is  united  with 
the  endomysium  of  the  surrounding  fibres.  The  muscle-substance 
is  never  directly  continuous  with  adjacent  tissues,  but  is  always 
enclosed  within  the  sac  of  the  sarcolemma ;  the  union  between  the 
fibres  and  other  structures  is  effected  by  the  blending  of  the  endo- 
mysium of  the  muscle-fibres  with  the  connective  tissue  of  the  attach- 
ments, whether  these  be  tendon,  periosteum,  perichondrium,  or 
subcutaneous  tissue ;  the  sarcolemma  closely  invests  the  sarcous 
contents,  being  simply  received  into  the  connective  tissue  without 
becoming  directly  continuous. 

CARDIAC   MUSCLE. 

The  muscular  tissue  of  the  heart,  as  well  as  of  the  cardiac  ends  of 
the  large  veins,  forms  an  intermediate  group  of  contractile  tissue, 
standing  in  its  development  between  the  simple 
spindle  non-striated  cell  on  the  one  hand  and 
the  highly  differentiated  striped  fibre  on  the 
other.  Among  the  lower  vertebrates  (fishes, 
amphibians)  the  cardiac  muscle  is  composed 
of  nucleated  spindle-cells  possessing  distinct 
transverse  striations  and  often  branched  ends ; 
in  man  and  the  higher  vertebrates  these  spindle- 
cells  give  place  to  short,  striated,  cylindrical 
fibres,  provided  with  lateral  processes.  By  the 
apposition  of  these  richly-branched  cells  a 
close,  narrow-meshed  net-work  is  formed,  the 
juncture  between  the  individual  elements  being 
indicated  by  transverse  lines  of  cement-sub- 
stance. 

The  peculiarities  of  heart-muscle  are — 
of   the    sarcolemma,    the    transversely   striated 
longitudinally   marked    muscular    tissue    being 


Heart  -  muscle,  showing 
several  joined  branched 
fibres :  around  the  poles  of 
the  nuclei  are  aggregations 
of  pigment-granules. 


The 


1.  ine  absence 
and  more  faintly 
naked. 

2.  The  situation  of  muscle-nuclei  within  the  sarcous  substance, 
usually  near  the  centre  of  the  cell. 

3.  The  characteristic  arrangement  of  the  contractile  fibrillae,  since 
these  are  so  placed  that  the  peripheral  fibrillae  are  grouped  into  flat, 
ribbon-like  muscle-columns,  somewhat  radially  disposed  about  the 
circumference  of  the  fibre  ;  the  remaining  central  portion  is  occupied 
by  prismatic  bundles  of  fibrillae,  together  with  the  nuclei  and  the 
associated  protoplasm  (Ranvier,  Kolliker).  The  small  masses  of  pro- 
toplasm which  surround  the  muscle-nuclei  usually  contain  minute 
fat-drops  and  pigment-granules.  The  amount  of  pigment  normally 
present  varies  with  age,   increasing  from  the  tenth  year  (Maass). 


Heart-muscle  fibres  in  sec- 
tion :  the  peripheral  zone 
is  composed  of  radially-ar- 
ranged groups  (/)  of  muscle- 
columns  ;  a  zone  (c)  of  less 
differentiated  sarcoplasm 
surrounds  the  nucleus. 


Fig. 


THE   MUSCULAR   TISSUES.  67 

Sometimes,  in  preserved  tissue,  the  position  of  the  nucleus  is  occu- 
pied by  a  clear  vacuole. 

Ranvier  has  called  attention  to  certain  differences  in  the  muscles 
of  the  rabbit,  describing  two  varieties — the  red  or  especially  dark 
(semitendinosus,  soleus)  and  the  white  or  pale 
(adductor  magnus).  The  red  muscles  are  char- 
acterized by  slow  response  to  electrical  stimulus, 
less  regular  transverse  striation,  greater  dis- 
tinctness of  longitudinal  markings,  and  great 
number  of  round  nuclei. 

The  blood-vessels   of  striated  muscle  are 
very  numerous.     The  larger  vessels,  together 
with  the  nerve-trunks  and,  less  frequently,  the 
lymphatics,  are  contained  within  the  perimy- 
sium, where  they  give  off  numerous  smaller 
branches ;  these,  in  turn,  extend  between  the  primitive  bundles  and 
break  up  into  extremely  thin  capillaries,  which  form  a  characteristic 
rectangular-meshed  net-work  around  the  indi- 
vidual muscle-fibres.     The  longer  sides  of  the 
meshes  correspond  with  the  axis  of  the  fibre. 
At  various   points  along  the   course   of  these 
vessels  peculiar  dilatations,  or  ampullae,  occur, 
the  object  of  which  is,  probably,  the  relief  of  sud- 
den temporary  interference  with  the  circulation 
during  muscular  contractions.     The  relation  be- 
tween the  capillary  blood-vessels  and  the  muscu- 
lar fibres  of  the  heart  is  very  intimate ;  in  many 
places  the  vessels  lie  embedded  within  or  even  en- 
tirely surrounded  by  the  muscular  tissue  (Meigs). 

Lymphatic  vessels  occur  in  striated  muscle 
in  small  numbers,  but  are  entirely  wanting  in 
many  small  muscles ;  when  distinct  lymphatic 
vessels  do  occur,  they  are  confined  to  the  larger 
and  looser  masses  of  the  perimysium  (Kolliker). 

The  nerves  supplying  the  striated  muscle 
include  the  principal  trunks  which  run  within  the  perimysium,  where 
they  subdivide  into  smaller  groups  of  medullated  fibres,  in  order  to 
reach  the  individual  muscle-fibres  ;  these  latter  receive  their  nervous 
supply  at  certain  points  only,  the  nerves  passing  to  the  muscle  to 
end  in  the  special  end-plates  in  the  manner  described  more  fully 
in  connection  with  the  peripheral  nerve-endings. 

The  development  of  all  varieties  of  muscular  tissue  is  closely 
related  to  the  mesoderm,  of  which  they  are  the  direct  descendants. 
The  plain  or  non-striated  muscle  is  formed  by  the  differentiation, 


Injected  voluntary  mus- 
cle :  the  capillaries  form 
rectangular-meshed  net- 
works enclosing  the  indi- 
vidual fibres. 


68 


NORMAL   HISTOLOGY. 


within  certain  areas,  of  the  irregular  mesodermic  elements  into  the 
elongated  fusiform  fibre-cells.  In  suitable  preparations  all  gradations 
between  the  ordinary  embryonal  connective-tissue  cells  and  the 
muscular  elements  may  be  observed,  emphasizing  the  common 
ancestry  of  the  two  forms  of  tissue. 

Voluntary  muscle,  representing  a  higher  specialization,  is  de- 
rived from  definite  areas  constituting  the  inner  layer  of  the  muscle- 
plates,  which  are  referable  to  the 
early  stages  of  the  primary  segmenta- 
tion into  somites.  The  cells  of  the 
muscle-plate  soon  elongate,  with  pro- 
liferation of  the  nuclei,  to  become  the 
primitive  muscle-fibres.  These  at 
first  consist  of  greatly  extended  ele- 
ments, possessed  of  numerous  nuclei 
and  composed  of  granular  indifferent 
protoplasm.  After  a  time  the  fibre 
exhibits  a  differentiation  into  longi- 
tudinal striae,  which,  later,  are  supple- 
mented by  the  transverse  markings 
characteristic  of  voluntary  muscle.  The 
sarcolemma  appears  about  the  time 
the  longitudinal  markings  are  seen. 

The  striations  are  limited,  at  first, 
to  one  side  of  the  fibre,  then  extend 
over  the  entire  periphery,  but  still  for 
some  time  do  not  reach  the  centre  of 
the  fibre,  an  inner  zone  of  undiffer- 
entiated sarcopiasm  occupying  the 
middle.  Later,  this  area  also  becomes 
converted  into  striated  tissue,  while  the 
once  numerous  nuclei  are  reduced  to  the 
few  collected  beneath  the  sarcolemma. 
Cardiac  muscle,  likewise,  develops 
from  the  mesoderm  immediately  surrounding  the  primary  heart-tubes, 
the  contractions  of  the  cells  being  displayed  even  before  the  histo- 
logical differentiation  becomes  apparent.  In  its  development  it 
represents  an  intermediate  stage,  since  the  original  spindle-cells  be- 
come converted  into  protoplasmic  fibres  containing  a  central  area 
which  always  remains  less  differentiated  and  nearer  its  primary  con- 
dition of  indifferent  sarcopiasm  than  the  peripheral  portions  of  the 
fibre.  The  fibres  of  Purkinje,  found  in  the  hearts  of  certain  rumi- 
nants, represent  muscular  fibres  in  which  the  sarcopiasm  remains  in 
part  still  undifferentiated. 


Developing  voluntary  muscle:  A, 
young  muscle-cells ;  a,  very  young 
spindle-cell ;  i,  older  element,  exhibiting 
indications  of  future  striation  on  one 
side ;  the  remaining  part  pf  the  cell  is 
composed  of  the  undifferentiated  sarco- 
piasm ;  B,  embryonal  muscle-fibres  pos- 
sessing many  nuclei  and  traces  of  striae ; 
C,  developing  muscle-fibres  in  section  ; 
in  the  larger  fibres  a  differentiated 
peripheral  zone  of  striae  (d)  is  seen  in 
section ;  an  area  of  still  indifferent  sar- 
copiasm occupies  the  centre  of  the  fibre 
and  surrounds  the  nucleus  («). 


THE   NERVOUS   TISSUES. 


69 


CHAPTER    V. 

THE    NERVOUS   TISSUES. 

The  nervous  system  is  composed  of  three  principal  parts — the 
tissues  originating  nervous  impulse,  the  nerve-cells  ;  the  structures 
serving  to  transmit  such  impulses,  the  nerve- fibres  ;  and  the  tissues 
uniting  and  supporting  the  nervous  elements,  the.  neuroglia  and 
connective-tissue  framework.  The  nerve-cells  are  the  primary 
elements,  being  older  in  the  development  of  the  individual  as 
well  as  in  the  evolution  of  the  nervous  system.  In  certain  inverte- 
brates both  generation  and  transmission  of  the  impulse  are  performed 
by  the  same  cell,  the  peripherally  situated  protoplasm  serving  to 
convey  and  expend  the  force  originating  within  the  more  centrally 
lying  parts  of  the  cell.  Such  simplicity,  however,  is  unusual,  the 
nerve-cell  soon  becoming  specialized  and  separated  from  the  pe- 
ripheral area  with  which  it  is  connected. 

NERVE-CELLS. 

Nerve-  or  ganglion-cells  of  man  and  other  vertebrates  differ  greatly 
in  form  and  size,  since  they  may  be  either  spherical  (Gasserian, 
spinal,  or  other  ganglia),  ellipsoidal  (spinal  cord),  pyriform  (cere- 
bellum), pyramidal  (cerebrum),  or  stellate  (spinal  cord),  and  vary 
from  10  to  100  fi  in  size  ;  the  huge  cells  of  the  spinal  cord  are  among 
the  largest  elements  of  the  body.  In  general  the  cells  of  motor 
areas  are  largest,  those  found  in  the  convolutions  bordering  the 
central  fissure  and  in  the  anterior  cornua  of  the  spinal  cord  being  of 
conspicuous  size. 

The  ganglion-cells  are  composed  of  granular  or  striated  proto- 
plasm, containing  a  large  round  or  oval  vesicular  nucleus  within 
which  lies  a  prominent  nucleolus  ;  after  certain  stains  the  protoplasm, 
nucleus,  and  nucleolus  present  distinct  tints.  Many  nerve-cells  are 
deeply  colored,  owing  to  the  presence  of  considerable  quantities  of 
pigment-granules  around  the  nucleus  ;  a  certain  amount  of  pigment 
within  the  protoplasm  is  almost  constant. 

The  protoplasm  of  every  nerve-cell  is  prolonged  into  at  least  one 
and  usually  several  processes,  dependent  upon  the  number  of  which 
it  is  customary  to  speak  of  nerve-cells  as  unipolar,  bipolar,  or 
multipolar.  Since  an  apolar  nerve-cell  is,  evidently,  functionally 
useless,  it  is  doubtful  whether  such  cells  ever  normally  exist ;  apolar 
cells  are  frequently  seen  in  preparations,  but  the  absence  of  the 


y0  NORMAL    HISTOLOGY. 

processes  is  only  apparent,  being  due  either  to  mutilation  or  to  the 
process  lying  without  the  plane  of  the  section  ;  where  processes  are 
really  wanting,  an  immature  or  pathological  condition  must  be 
suspected. 

The  processes  of  nerve-cells  are  of  two  principal  kinds — the 
branched  protoplasmic  and  the  axis-cylinder  (Deiters's)  pro- 
cesses. When  a  cell  possesses  but  one,  this  is  always  an  axis- 
cylinder  process.  The  proto- 
plasmic processes  rapidly 
undergo  dichotomous  di- 
vision, splitting  up  and  sub- 
dividing until  the  resulting 
branches  form  rich  net-works 
or  arborizations  of  slender 
threads,  which  frequently 
interlace,  but  probably  never 
actually  join,  with  similar 
fibrils  of  adjacent  cells. 
Nerve-cells,  in  one  sense,  are 
but  nucleated  local  accumula- 
tions of  the  interfibrillar  pro- 
toplasm, which  latter  may  be 
termed  neuroplasm  (K61- 
liker) ;  the  large  striated 
multipolar  ganglion-cells  may 
be  regarded  as  switch-boards 
for  the  redistribution  of  the 
numerous  ultimate  fibrillae 
continued  into  the  axis-cylin- 
ders. The  fibrillae  pass  off  in 
divergent  paths,  along  the 
several  processes  of  the  cell, 
to  form  new  combinations  and 
relations. 

The  peculiarities  formerly  supposed  to  constitute  the  distinguish- 
ing characteristics  of  the  axis-cylinder  processes  are  no  longer  suf- 
ficient in  the  light  of  recent  advances  in  our  knowledge  regarding  the 
structure  of  the  nervous  system.  The  investigations  of  Golgi  and 
others  have  shown  that,  in  addition  to  greater  delicacy  and  a  straighter 
course,  the  axis-cylinder  processes  present  variations  which  separate 
ganglion-cells  into  two  groups — cells  of  the  first  and  cells  of  the 
second  type. 

Nerve-cells  of  the  first  type  include  elements,  as  those  of  the 
motor  areas,  possessing  the   characteristic   axis-cylinder   processes 


Nerve-cell  from  the  cerebral  cortex,  exhibiting  the 
striations  of  the  protoplasm  and  the  conspicuous  char- 
acter of  the  nucleus  and  the  nucleolus :  /,  pigment- 
granules  :  a,  axis-cylinder  process  ;  b,  I,  apical  and 
lateral  protoplasmic  processes. 


THE   NERVOUS   TISSUES. 


In- 


directly continuous  with  the  axis-cylinder  of  the  nerve-fibre.  While 
these  processes,  when  compared  with  the  richly-divided  protoplasmic, 
may  be  regarded  as  unbranched,  the  existence  of  delicate  lateral  off- 
shoots, or  collateral  fibrils,  has  been  established ;  these  delicate 
branches  pass  backward  towards  the  gray  matter,  within  which  they 
end. 


Nerve-cell  from  the  spinal  cord,  isolated  by  maceration  and  teasing  ;  the  numerous  branched  pro- 
toplasmic processes  are  somewhat  displaced  and  distorted,  owing  to  manipulation  :  a,  axis-cylinder 
process. 

Nerve-cells  of  the  second  type  are  distinguished  by  the  be- 
havior of  the  axis-cylinder  process  ;  this,  instead  of  passing  into 
the  white  matter  to  become  the  centre  of  a  nerve-fibre,  never  leaves 
the  gray  matter  in  which  the  ganglion-cell  lies,  but,  after  a  longer 
or  shorter  course,  rapidly  undergoes  division  arid  subdivision  in  the 
production  of  a  terminal  arborization  of  delicate  fibrillae  ;  these 
ramifications  are  limited  entirely  to  the  gray  matter,  their  exact 
manner  of  ending  and  their  relations  to  other  cells  varying  in  dif- 
ferent parts.  The  free  division  of  the  axis-cylinder  process  does 
not  curtail  the  branching  of  the  protoplasmic  extensions,  which  are 
often  very  conspicuous,  notwithstanding  the  numerous  bifurcations 
of  the  former.  In  some  instances  the  axis-cylinder  processes  of 
cells  of  this  type  split  up  into  fibrils  which  enclose  the  bodies  of 
other  nerve-cells  within  basket-like  net-works  ;  a  notable  ex- 
ample of  this  arrangement  exists  in  the  cerebellum  around  the  cells 
of  Purkinje. 


72 


NORMAL   HISTOLOGY. 


Fig.  84. 


The  axis-cylinder  processes  usually  are  directed  towards  the 
nearest  mass  of  white  matter,  since  the  axis-cylinder  of  the  nerve- 
fibre  becomes  continuous  with  that  of  the  cell.  Exceptional 
arrangements   are   sometimes   encountered,  as  where  one   process 

of  a  bipolar  cell  becomes  wound  about 
the  remaining  straighter  fibre,  con- 
stituting a  spiral  process  ;  such  cells 
are  comparatively  frequent  in  the  sym- 
pathetic ganglia  of  the  frog. 

Ganglion-cells  lie  within  peri-cellu- 
lar lymph-spaces,  which  appear  with 
greater    or    less 
Fig.  85.  distinctness     ac- 

i/         cording    to    the 
condition  of  the 


Nerve-cell  of  first  type — from  cere- 
bral cortex :  /',  /,  protoplasmic  pro- 
cesses directed  respectively  towards  the 
free  surface  and  laterally  ;  a,  axis-cylin- 
der or  nerve-process  giving  off  collateral 
branches,  c,  c.    Golgi  staining. 


Nerve-cell  of  second  type — 
from  cerebellum  :  /,  branched 
protoplasmic  processes  ;  c,  cell- 
body  ;  a,  axis-cylinder  process 
breaking  up  into  arborization 
(»),  but  entirely  confined  to 
gray  matter.     Golgi  staining. 


Basket  -  work,  formed 
by  the  extensions  of  the 
branched  axis -cylinder 
process  of  a  nerve-cell, 
surrounding  the  body  of 
one  of  the  ganglion-cells 
of  Purkinje  :  p,  base  of 
branched  process  of  Pur- 
kinje's  cell ;  n,  fibrils  con- 
stituting basket-work. 


protoplasm  of  the  enclosed  cell ;  when  this  is  contracted  and 
shrunken  the  space  is,  obviously,  more  conspicuous  than  when  almost 
entirely  filled  by  the  cell.  These  lymph-spaces  are  limited  by 
a  delicate,  elastic,  hyaline  membrane,  and  lined  with  nucleated 
endothelial  plates ;  on  the  exit  of  the  axis-cylinder  a  delicate 
prolongation  of  this  sheath  accompanies  the  fibre  as  the  neuri- 
lemma. 


NERVE-FIBRES. 

Depending  upon  the  character  of  the  investing  coats,  nerve-fibres 
appear  as  two  kinds — the  medullated,  or  white,  and  the  non- 
medullated,  or  gray.  These  do  not,  however,  constitute  two 
sharply  defined  and  distinct  classes,  but  depend  upon  variations  in 
the  condition  of  fibres,  which  often  represent  both  varieties  at  dif- 


THE   NERVOUS   TISSUES. 


73 


ferent  portions  of  their  course.  Every  medullated  nerve-fibre  loses 
its  white  substance  of  Schwann  and  becomes  non-medullated  before 
reaching  its  ultimate  distribution.  The  majority  of  nerve-fibres 
constituting  the  great  cerebro-spinal  tract  may  be  classed  as  med- 
ullated, although  numbers  of  gray  fibres  likewise  occur  here ;  the 
non-medullated  fibres  are  especially  numerous  in  the  sympathetic 
system,  where  they  predominate,  as  well  as  in  certain  of  the  cranial 
nerves,  as  the  olfactory.  While  the  character  of  the  fibre,  as  to 
whether  it  is  motor  or  sensory,  bears  no  relation  to  its  size,  the 
length  of  the  fibre  seems  to  directly  influence 
its  diameter,  since  fibres  having  long  courses 
possess  greater  width  than  those  extending  for 
much  shorter  distances. 

A  typical  medullated  nerve-fibre  consists  of 
the  following  parts  : 

1.  The  axis-cylinder,  surrounded,  possibly, 
by  its  sheath,  or  axilemma  (Kiihne). 

2.  The  medullary  substance,  or  white  matter 
of  Schwann. 

3.  The  neurilemma,  or  sheath  of  Schwann, 
with  the  nerve-corpuscles. 

Perfectly  fresh,  uninjured,  medullated 
nerve-fibres,  when  examined  by  transmitted 
light,  appear  as  homogeneous,  hyaline  cylinders, 
with  dark  contours  and  no  appreciable  structure  ; 
seen  by  reflected  light,  the  fatty  character  of  the 
medullary  substance  is  indicated  by  the  glisten- 
ing appearance  of  the  fibres,  and  their  dull  white 
color  when  viewed  in  masses.  Shortly  after 
death  the  fibres  exhibit  characteristic  double 
contours,  enclosing  an  apparently  structureless 
centre ;  later,  the  fibres  become  mottled  by 
irregular  spherical  masses,  derived  from  the  dis- 
torted medullary  substance. 

The  axis- cylinder  appears,  in  fresh  nerves  or 
in  those  fixed  with  osmic  acid  and  teased,  as  an 
inconspicuous,  clear,  delicate  rod  extending  along  the  central  part 
of  the  fibre,  or,  perhaps,  projecting  beyond  the  outer  sheaths  at 
the  broken  end.  The  longitudinal  striations  occasionally  seen, 
under  high  amplification,  in  carefully  fixed  preparations,  are  indi- 
cations of  the  ultimate  fibrillae  of  which  the  axis-cylinder  is  com- 
posed ;  these  fibrillae  are  cemented  together  by  a  finely  granular, 
interstitial  substance,  or  neuroplasm  (Kblliker).  According  to 
Kiihne,  the  axis-cylinder  is  enveloped  by  a  special,  delicate,  elastic 


Nerve-cell  from  a  sym- 
pathetic ganglion  of  frog, 
showing  the  tortuous  course 
and  terminal  net-work  of 
the  spiral  fibre:  n,  neuri- 
lemma continued  as  a  deli- 
cate sheath.  (After  Ket- 
zius.) 


_.  NORMAL    HISTOLOGY. 

74 

sheath — the  axilemma  (Kiihne)  ;  other  authorities  regard  this 
appearance  as  an  artihcial  production.  Since  every  axis-cylinder 
is  connected  with  the  corresponding  process  of  a  nerve-cell, 
axis-cylinders    may   be    regarded    as    direct    continuations   of   the 

gang  lion- cells,    their    component    fibrillae 
forming   uninterrupted    paths  which  con- 
nect   the    periphery    with    the    presiding 
nerve-centres.     On    approaching    its   ter- 
mination,    the    axis-cylinder    splits    into 
smaller  bundles  of  component  fibrillae ; 
these   groups   subsequently  divide,   until, 
finally,    the    naked    ner- 
vous   threads,    singly    or 
in    small    groups,    reach 
their  ultimate  destination. 
The  nerve-fibrillae  not  in- 
frequently     exhibit      nu- 
merous   minute    fusiform 
enlargements      or     vari- 
cosities      along       their 
course,     giving     to     the 
fibrils       a      characteristic 
beaded     appearance,     es- 
pecially  after    gold-stain- 
ing. 

The  medullary  sub- 
stance, or  white  matter 
of  Schwann,  surrounds  the  axis-cylinder,  and  forms  the  most  con- 
spicuous investment  of  the  fibre.  The  existence  of  a  narrow  lym- 
phatic cleft  described  as  lying  between  the  medullary  substance  and 
the  axilemma  is  still  uncertain.  The  medullary  substance  consists 
of  two  parts :  one  of  these  occurs  as  a  delicate  reticulated  frame.- 
work,  composed  of  a  resistant  material  probably  resembling  neuro- 
keratin (Kiihne  and  Ewald)  ;  the  other  fills  the  interstices  of  the 
reticulum  and  appears  as  a  semi-fluid,  highly  refracting,  fatty  sub- 
stance—the myelin— which  affords  protection  to  the  enclosed  axis- 
cylinder.  Other  authorities  regard  the  reticulated  framework  as 
the  effect  of  reagents,  citing  the  variability  in  the  appearances  of  the 
net-work  as  opposed  to  its  presence  as  a  normal  constituent  of  the 
coat. 

At  regular  intervals  along  the  medullated  nerve-fibres  well-marked 
annular  constrictions  occur ;  these  are  the  nodes  of  Ranvier,  and 
mark  the  interruption  of  the  white  substance  of  Schwann  at  certain 
points. 


Medullated  nerve-fibres :  A , 
teased  in  salt  solution,  x,  shortly 
after  death  ;  y,  a  node  of  Ranvier  ; 
z,  post-mortem  distortions  of  med- 
ullary substance.  B,  an  isolated 
stained  fibre;  a,  axis-cylinder ;  r, 
node  of  Ranvier;  m,  medullary 
substance  ;  n,  neurilemma,  beneath 
which  a  nerve-corpuscle  is  seen  in 
the  lower  segment. 


Gold-stained  axis- 
cylinder  (a),  showing 
component  fibrillae ; 
b,  varicose  nerve- 
fibrillas  near  their 
termination. 


THE   NERVOUS   TISSUES. 


75 


Fig.  90. 


Owing  to  the  absence  of  the  middle  coat  in  these  positions,  the 
outer   sheath,    or   neurilemma,    is    brought    into    contact   with   the 
continuous  axis-cylinder.     The  portions  of  the  fibre  included  be- 
tween  two   constrictions — the  internodes,   or 
internodal    segments — vary     in     length     with 
the    size    of   the    fibre,    being    longer    (about 
1    mm.)    in   large   and    much   shorter   in   thin 
fibres.     Each  internode  pos- 
sesses  a  single    nerve-cor- 
puscle,    usually     about     its 
middle,    and    probably   elon- 
gates  during   the   growth  of 
the  nerve.     The  neurilemma 
is  not  broken  by  the  nodes 
into   segments,    but   forms   a 
continuous     sheath.       When 
a      medullated      nerve  -  fibre 
branches,  the  bifurcation  cor- 
responds in  position  to  a  node 
of  Ranvier. 

After  treatment  with  silver 
nitrate  the  positions  of  the 
nodes  of  Ranvier  are  rendered 
conspicuous  by  the  appear- 
ance of  minute  dark-brown 
crosses  ;  the  transverse  arm 
is  formed  by  the  stained, 
internodal  albuminous  sub- 
stances, forming  an  annular  disk,  sometimes  called  the  constricting 
band  (Ranvier),  while  a  stained  portion  of  the  axis-cylinder  con- 
tributes the  less  distinctly  marked  vertical  lines  of  the  cross. 
Closely-placed  transverse  markings,  known  as  Frommann's  lines, 
as  well  as  bi-conical  swellings,  occasionally  are  noted  along  the  axis- 
cylinder  after  treatment  with  silver ;  their  significance,  however,  is 
still  undetermined. 

The  medullary  substance  is  very  prone  to  post-mortem  change, 
the  coagulated  or  partly  disintegrated  myelin  producing  various 
grotesque  distortions  in  the  contour  of  the  nerve-fibre.  After  treat- 
ment with  osmic  acid  and  other  reagents,  the  white  substance  of 
Schwann  displays  oblique  markings  which  are,  apparently,  clefts  or 
incisions  involving  the  middle  coat ;  relying  upon  these  appearances, 
many  regard  the  medullary  substance  as  made  up  of  elongated  pieces, 
the  Schmidt-Lantermann  segments,  several  of  which  are  in- 
cluded within  each  internode. 


Medullated  nerve- 
fibres  after  treatment 
with  osmic  acid,  from 
frog  :  A  ,  fibre  displays 
the  incisions  of  the  me- 
dulla, or  Schmidt-Lan- 
termann segments ;  B, 
the  medullary  substance 
exhibits  a  reticulated 
appearance. 


Silvered  nerve-fibres :  A, 
small  bundle  of  medullated 
fibres  displaying  the  silver 
crosses  at  several  nodes  ;  B, 
node  of  Ranvier  under  high 
power  :  the  horizontal  limb 
of  the  cross  is  produced  by 
the  stained  intersegmental 
cement-substance ;  the  ver- 
tical limb  is  formed  by  the 
colored  axis-cylinder;  C, 
silvered  axis-cylinder  show- 
ing a  bi-conical  enlarge- 
ment and  the  transverse 
markings  or  lines  of  From- 
mann. 


Fig 


-5  NORMAL   HISTOLOGY. 

The  neurilemma,  sheath  of  Schwann,  or  primitive  sheath, 
the  outer  covering  of  the  nerve-fibre,  is  a  delicate,  homogeneous, 
elastic  membrane,  closely  investing  the  medullary  substance,  and 
resembling  the  sarcolemma.  On  its  inner  surface,  placed  at  regular 
intervals  corresponding  to  the  position  of  the  nodes  of  Ranvier,  are 
the  nerve-corpuscles,  meagre  accumulations  of  protoplasm  sur- 
rounding the  oval  nuclei.  The  medullated  fibres  of  the  white  matter 
of  the  brain  and  spinal  cord,  as  well  as  those  composing  the  optic 
and  acoustic  nerves,  are  noteworthy  as  being  without  a  neurilemma, 
the  surrounding  neuroglia  in  these  positions  assuming  the  support 
and  covering  of  the  fibres. 

The  non-medullated,  pale,  or  Remak's  fibres,  as  indicated  by 
the  first  name,  are  devoid  of  medullary  substance,  consisting  of  the 
axis-cylinder  and  the  more  or  less  modified  neuri- 
lemma ;  such  fibres,  when  aggregated,  appear  as 
grayish,  semi-transparent  bands.  While  every 
medullated  nerve-fibre,  before  reaching  its  pe- 
ripheral distribution,  loses  the  white  substance  of 
Schwann  and  becomes  sooner  or  later  a  non- 
medullated  fibre,  the  nerves  constituting  the 
sympathetic  system  especially  represent  this 
group,  and  evince  the  distinctive  tendency  to 
give  off  branches,  which  unite  to  form  the  char- 
acteristic plexuses.  The  presence  of  both  va- 
rieties of  fibres,  however,  in  nerve-trunks  is 
quite  usual ;  a  conspicuous  example  of  this  asso- 
ciation is  found  in  the  vagus  of  the  dog,  where 
large  bundles  of  both  kinds  are  included  within 
a  common  sheath. 

The  fibrillse  constituting  the  axis-cylinders  of 
non-medullated  fibres  are  especially  distinct,  this 
feature  being  probably  due  to  the  generous 
amount  of  neuroplasm  separating  the  fibrillse  ;  not  infrequently  local 
accumulations  of  this  interfibrillar  substance  occur,  producing  the 
conspicuous  varicosities  seen  along  the  course  of  the  fibres.  The 
nerve-nuclei  are  far  more  numerous  than  in  medullated  fibres  ;  they 
are,  however,  irregularly  distributed,  lying  upon  the  surface  of  the 
fibre  and  beneath  the  outer  delicate  sheath.  This  enveloping  sheath 
— the  attenuated  representative  of  the  neurilemma — is  often  difficult 
or  impossible  to  distinguish,  being  very  thin  and  closely  adherent  to 
the  fibre.  The  smallest  nerve-fibrils  are  probably  without  this  coat, 
the  fibrillas  continuing  as  naked  bundles,  with  the  exception  of  the 
imperfect  covering  afforded  by  the  numerous  overlying  nerve-nuclei. 
Non-medullated  nerve-fibres  are  prone  to  form  rich  plexuses,  the 


Non-medullated  nerve- 
fibres  from  the  sympa- 
thetic system  :  the  nucle- 
ated fibres  join  to  form  a 
plexus. 


THE   NERVOUS   TISSUES. 


77 


junction  of  several  fibres  being  frequently  marked  by  characteristic 
triangular  areas,  in  which  a  number  of  the  nerve-nuclei  are  often 
collected. 


Fig 


THE    NERVE-TRUNKS. 

The  nerve-fibres  already  described  are  associated  in  bundles — the 
funiculi — which,  in  turn,  may  be  grouped  to  constitute  the  large 
macroscopic  nerve-trunks.  The 
funiculi  differ  greatly  in  diameter,  a 
number  of  varying  size  being  usually 
included  within  the  nervous  cord ; 
in  very  small  nerves,  however,  a 
single  funiculus  may  suffice  to  form 
the  entire  trunk.  While  both  kinds 
of  fibres  are  grouped  in  bundles, 
the  nerves  composed  principally  of 
medullated  fibres  present  the  more 
typical  arrangement. 

On  transverse  section  of  such 
trunks  the  individual  nerve-fibres 
appear  as  small,  round,  nucleated 
cells,  whose  somewhat  eccentrically 
placed  nuclei  are  the  axis-cylinders 
in  section,  while  the  contours  of  the 
cell-like  areas  are  formed  by  the 
sections  of  the  neurilemma ;  the 
shrunken  granular  or  concentrically 
marked  masses  within  the  apparent 
cell-walls  are  the  remains  of  the 
medullary  substance.  These  sec- 
tions of  the  fibres  are  held  in  place 

by  a  delicate  connective  tissue — the  endoneurium — extending 
among  and  surrounding  the  individual  fibres.  When  the  nerve- 
bundle,  or  funiculus,  is  small,  the  nerve-fibres  are  uniformly  dis- 
tributed, and  it  is  spoken  of  as  simple  ;  when  large,  however,  the 
fibres  are  usually  divided  into  irregular  groups  by  stronger  fibrous 
trabeculae,  thus  forming  a  compound  funiculus.  The  individual 
nerve-fibres  vary  greatly  in  diameter  (from  2  to  20  /*),  even  adjoining 
fibres  often  exhibiting  marked  differences.  In  general,  the  cerebro- 
spinal nerves  possess  the  largest  fibres,  the  sympathetic  much  the 
smallest  (2  to  4  tx),  while  the  components  of  many  of  the  cranial 
nerves  occupy  an  intermediate  position. 

Each  funiculus  is  invested  by  a  robust  connective-tissue  sheath — 
the  perineurium — between  the  fibrous  lamellae  of  which  are  seen 


Section  of  portion  of  a  nerve-trunk  including 
three  bundles,  or  funiculi,  surrounded  by  the 
perineurium  (/) ;  the  funiculi,  together  with 
the  blood-vessels  and  adipose  tissue,  are 
united  by  the  more  general  epineurium  (e) ; 
the  sections  of  the  individual  nerve-fibres  are 
held  in  place  by  the  endoneurium  ;  f,  fat-cells, 
near  which  are  the  sections  of  blood-vessels. 


Fig 


«g  NORMAL   HISTOLOGY. 

the  nuclei  of  the  endothelioid  plates  lying  within  the  interlamellar 
lymphatic  spaces.  The  endoneurium  is  directly  continuous  with  the 
perineurium,  of  which  it  is  the  intrafunicular  extension. 

Where  a  nerve-trunk  comprises  several  funiculi,  these  are  held 
together  and  enveloped  by  a  loose  general  connective  tissue — the 
epineurium — which  supports  the  blood- 
vessels and  lymphatics,  and  often  contains 
masses  of  adipose  tissue  ;  the  external  layer 
of  the  epineurium  is  usually  somewhat 
condensed. 

When    the    funiculus    divides,    the   new 
bundles  receive  a  prolongation  of  the  peri- 
neurium, the  investment  becoming  thinner 
with  each  successive  division.     On  nearing 
their  final  destination,  the  funiculi  break  up 
into  small  groups  or  single  fibres,  which  are 
covered  by  an  attenuated  extension  of  the 
formerly  robust  perineurium  ;    this  invest- 
ment constitutes  the  sheath  of  Henle,  and 
consists  of  a  delicate  fibrous  envelope  lined 
with  endothelioid  plates  ;  in  some  cases  these 
latter  alone  represent  the  entire  sheath. 
The  larger  blood-vessels  enclosed  within 
the  epineurium  give  off  branches,  which  surround  the  funiculi  and 
break  up  into  capillaries  passing  within  the  endoneurium  among  the 
fibres. 

The  lymphatics  are  represented  by  irregular  clefts  within  the 
endoneurium,  which  are  connected  with  the  interlamellar  spaces  of 
the  perineurium  ;  from  these  the  lymph  is  taken  up  and  carried  off 
by  the  more  definite  lymphatic  channels  running  within  the  epineu- 
rium. 

The  nerves  of  the  larger  trunks — the  nervi  nervorum — are  dis- 
tributed within  the  epineurium,  and  are  said  to  terminate,  in  many 
cases,  in  special  bodies  which  resemble  in  general  type  the  spherical 
end-bulbs  of  Krause. 


A  single  funiculus  more  highly 
magnified ;  the  apparent  small 
nucleated  cells  are  sections  of  the 
nerve-fibres  and  their  axis-cylin- 
ders :  a,  axis-cylinder ;  w,  med- 
ullary substance  ;  «,  neurilemma  ; 
e ,  endoneurium  ;  /,  perineurium  ; 
b,  connective-tissue  cells  of  same. 


THE   SUPPORTING   TISSUES   OF   THE   NERVE-CENTRES. 

The  essential  constituents  of  the  nervous  system,  the  cells  and 
fibres,  when  associated  in  large  masses,  as  in  the  cerebro-spinal  tract, 
are  held  in  place  and  supported  by  two  varieties  of  sustentacular 
tissue.  On  examining  suitably  prepared  sections  of  these  organs, 
the  cells  and  fibres  appear  everywhere  to  be  embedded  within  a  finely 
reticulated  ground-substance,  whose  composition  is  especially 
complex  in  the  gray  matter.     The  basis  of  this  reticulum  is  the 


THE   NERVOUS   TISSUES. 


79 


Supporting  tissues  of  nerve-centres  :  A ,  extensions  of  the 
peripheral  connective  tissue  of  the  pia  mater  ;  B,  neuroglia- 
cells,  one  of  which  is  seen  in  profile  (s).     Golgi  staining. 


neuroglia,  a  peculiar  form  of  ectodermic  tissue,  with,  therefore, 
close  relations  to  the  neurogenetic  tract.  Neuroglia  consists  of 
extremely  branched  elements,  or  glia- cells,  whose  numerous  pro- 
cesses   break    up     into 

brush-like     bundles     of  Fig.  95. 

delicate     fibrils,     which  A  ^^,  B 

pass  in  all  directions 
among  the  nervous  ele- 
ments, filling  more  or 
less  completely  all  inter- 
stices. The  body  of  the 
glia-cells  is  frequently 
stellate,  possessing  a  nu- 
cleus and  staining  in- 
tensely with  certain  dyes. 
The  demonstration  of 
these  neuroglia  elements 
is  very  striking  in  Golgi 

silver  preparations,  where  they  appear  as  dark,  spider-like  figures 
which  send  out  delicate  fibrils  in  all  directions.  In  the  gray  matter 
the  ground-reticulum  is  composed  of  the  minutely  ramifying  ter- 
minal threads  of  the  processes  of  the  nerve-cells,  the  axis-cylinders 
of  the  nerve-fibres,  together  with  the  extensions  of  the  neuroglia 
elements.  The  groundwork  surrounding  the  nerve-fibres  within 
the  white  matter  serves  the  purpose  of  covering  as  well  as  of  support, 
and  replaces  the  neurilemma. 

In  addition  to  the  dense  reticulum  formed  by  the  neuroglia,  con- 
stituting the  special  sustentacular  tissue  of  the  nervous  system,  pro- 
longations from  the  enveloping  pia  mater  likewise  penetrate  within 
the  nervous  masses  and  contribute  connective-tissue  trabeculae, 
which  form  a  supporting  framework  throughout  the  organs.  These 
connective-tissue  ingrowths  constitute  the  septa,  which  in  many 
places,  as  conspicuously  in  the  spinal  cord,  separate  the  nervous 
matter  into  distinct  tracts  and  areas.  The  finer  ramifications  of  these 
partitions  fade  away  in  delicate  extensions  which  mingle  with  the 
fibrils  of  the  neuroglia-cells.  It  is  evident,  therefore,  that  the  sup- 
porting tissue  of  the  nervous  system  can  no  longer  be  regarded 
simply  as  a  form  of  connective  tissue,  since,  in  addition  to  the  un- 
doubted connective  tissue  present,  the  larger  part  is  contributed  by 
the  peculiar  ectodermic  structure,  the  neuroglia. 

THE  STRUCTURE  OF  GANGLIA. 

Along  the  course  of  certain  nervous  cords,  such  as  those  consti- 
tuting the  sensory  roots  of  the  spinal  nerves,  the  trunks  of  many  of 


8o 


NORMAL   HISTOLOGY. 


the  cranial  nerves,  and  especially  of  the  sympathetic  system,  groups 
of  nerve-cells  occur  associated  with  the  nerve-fibres  in  the  form  of 
ganglia ;  these  may  be  large  and  conspicuous  masses,  as  the  Gasserian 


Fig.  96. 


Spinal  ganglion,  in  longitudinal  section,  from  cat :  the  groups  of  nerve-cells  lie  embedded  among  the 
bundles  of  the  nerve-fibres. 

ganglion  of  the  trifacial  nerve,  or  their  size  may  be  microscopic,  as 
many  of  the  interstitial  ganglia  connected  with  the  distribution  of 
the  sympathetic  fibres. 

The  outer  covering  of  the  ganglion  consists  of  a  fibrous  envelope, 
a  condensation  of  the  adjacent  epineurium  in  many  cases,  from  which 

prolongations  extend  among 
Fig.  97.  the  nervous  elements,  where 

they  break  up  into  delicate 
bundles  of  connective  tissue, 
which  serve  for  the  union 
and  the  support  of  the  cells 
and  the  fibres.  Some  of  the 
nerve-fibres  pass  through 
the  ganglion  on  their  way 
to  more  distant  points  with- 
out joining  any  of  the  nerve- 
cells,  while  many  others  end 
in  or  take  origin  from  these 
elements.  The  presence  or 
absence  of  the  medullary 
coat  depends  upon  the  char- 
acter of  the  component 
fibres  of  the  nerve-trunk ; 
before  joining  a  nerve-cell, 
however,  the  medullary  substance  disappears,  while  the  neuri- 
lemma of  the  fibre  continues  and  becomes  the  nucleated  capsule 


Section  of  spinal  ganglion  more  highly  magnified  : 
g,  the  nerve-cells,  cut  in  various  planes,  surrounded 
by  the  nucleated  sheath  (c) ;  a,  the  medullated  nerve- 
fibres,  on  which  several  nodes  of  Ranvier  are  seen  ;  b, 
cells  of  the  supporting  connective  tissue. 


THE   NERVOUS   TISSUES. 


81 


Fig.  98. 


enclosing  the  individual  nerve-cells.  These  latter  possess,  in 
general,  a  spherical  form,  and  are  usually  provided  with  one  or  two, 
seldom  more,  processes  ;  in  the  bipolar  cells  the  processes  frequently 
pass  from  opposite  poles  to  become  continuous  with  the  afferent 
and  efferent  fibres.  In  the  ganglia  of  some  of  the  lower  vertebrates 
bipolar  cells  occur  in  which  one  process  becomes  invested  by  the 
turns  of  the  other  or  spiral  fibre.  Unipolar  cells  exist  in  which  the 
single  process  divides  into  T-branches  extending  almost  at  right 
angles  ;  such  cells  occur  also  in  man. 

The  development  of  all  nerve-fibres  and  nerve-cells  must  be 
referred  to  the  elements  derived  from  the  invaginated  ectoderm 
forming  the  neural  tube.  Without  entering 
upon  an  exhaustive  account  of  the  process, 
many  details  of  which  are  still  uncertain,  it  may 
be  accepted  that  the  primary  neural  ectoderm 
differentiates  into  two  varieties  of  cells — the 
neuroblasts  and  spongioblasts.  The  nerve- 
fibres  are  formed  as  outgrowths  from  the  primi- 
tive nerve-cells  or  neuroblasts.  This  may  take 
place  either  in  one  direction  alone,  from  the 
centre  towards  the  periphery  {centrif tig  ally),  as 
in  the  formation  of  the  efferent  fibres  of  the 
motor-nerve  roots  of  the  spinal  cord ;  or,  as  in 
the  production  of  the  afferent  (sensory)  nerves, 
the  neuroblasts  may  be  somewhat  removed  from 
the  central  nervous  mass,  occupying  the  position 
of  the  spinal  ganglia,  and  send  out  fibre-pro- 
cesses in  two  directions,  one  set  growing  into 
the  nerve-centre  {centripetal ly),  while  a  second 
group  of  fibres  extends  towards  the  periphery' 
{centrifngally).  In  all  cases  the  nerve-fibres 
are  formed  as  outgrowths  from  the  primary 
nerve-cells;  in  later  stages  the  cells  concerned 
in  extending  the  nervous  path  may  disappear 
after  the  establishment  of  the  tract.  The  spongioblasts,  on  the 
other  hand,  are  especially  concerned  in  the  production  of  the  neuro- 
glia-cells,  these  ultimately  becoming  transformed  into  the  close  reticu- 
lar formation  supporting  the  nervous  elements. 

The  nerve-fibres  are  at  first  pale  and  possess  neither  medullary 
substance  nor  neurilemma.  The  acquisition  of  the  white  substance 
of  Schwann  occurs  much  later,  the  exact  mode  of  its  production, 
however,  being  by  no  means  certain ;  whether  the  medullary  sub- 
stance owes  its  formation  to  the  influence  of  the  axis-cylinder,  or  its 
origin  must  be  referred  to  the  more  or  less  direct  agency  of  the  ele- 

6 


Ganglion  nerve-cell  with 
spiral  fibre  from  the  sym- 
pathetic of  frog  :  Sf>  F,  the 
spiral  fibre  surrounding  the 
straight  process  (Cf)  and 
dividing  at  a  node  of  Ran- 
vier  ( Th) ;  s,  neurilemma. 
(After  Schieffer decker .) 


g2  NORMAL   HISTOLOGY. 

ments  represented  by  the  nerve-nuclei,  future  investigation  must 
determine.  If  traced  to  the  axis-cylinder,  the  sheath  must  be  classed 
as  ectodermic  tissue ;  as  mesodermic,  on  the  other  hand,  if  referred 
to  the  nerve-corpuscles.  The  period  at  which  the  medullary  coat 
appears  in  the  various  groups  of  nerves  is  variable,  but  constant  for 
given  tracts  ;  account  has  been  taken  of  this  fact  with  great  advantage 
in  the  laborious  investigations  of  tracing  the  path  of  many  nerve- 
tracts  composed  of  medullated  fibres.  The  neurilemma  may  be 
regarded  as  certainly  derived  from  the  differentiation  of  surrounding 
mesodermic  cells,  as  likewise  the  more  general  connective-tissue 
envelopes  constituting  the  endoneurium,  the  perineurium,  and  the 
epineurium  of  the  nerve-bundles. 


THE   PERIPHERAL   NERVE-ENDINGS. 


83 


CHAPTER    VI. 


THE    PERIPHERAL    NERVE-ENDINGS. 

Terminations  of  Sensory  Nerves.  A  medullated  nerve,  in 
passing  to  its  ultimate  distribution,  first  loses  the  medullary  sub- 
stance, or  white  matter  of  Schwann,  which  ends  abruptly  at  some 
bifurcation  of  the  nerve  corresponding  in  position  to  a  node  of  Ran- 
vier.  The  fibre  continues  for  a  variable  distance  non-medullated, 
being  covered  with  the  neuri- 
lemma and  the  nerve-cor-  FlG-  99- 
puscles ;  these  coats  become 
reduced  gradually  until  the 
neurilemma  disappears,  the 
nerve-nuclei  then  alone  re- 
maining as  an  imperfect  in- 
vestment of  the  axis-cylinder. 
The  nuclei  soon  occur  less 
frequently,  until  finally  they 
disappear  and  the  bundles  of 
nerve-fibrillae,  by  this  time 
greatly  reduced  owing  to  re- 
peated division,  continue  as 
naked  axis-cylinders ;  these 
unite  to  form  a  widely-meshed 
ground-plexus,  possessing 
characteristic  triangular,  nu- 
cleated nodal  points  where 
the  bundles  of  fibrillar  meet. 

The  axis-cylinders  sooner 
or  later  break  up  into  their 
component  primitive  fibrillse,  which  unite  with  one  another  to 
form  rich  net-works,  or  terminal  plexuses,  within  the  connective 
tissue  of  the  organ  supplied  ;  these  net-works  quite  often  are  situ- 
ated immediately  beneath  the  epithelium  ;  in  immediate  proximity 
with  the  basement  membrane  fine  fibrillae  emerge  from  the  plexus, 
enter  the  epithelium,  and  terminate  in  pointed  or  club-shaped  free 
endings  between  the  epithelial  cells.  The  nerves  of  common  sen- 
sation frequently  end  in  this  manner,  including,  probably,  many 
nerves  of  the  skin,  cornea,  and  mucous  membranes. 

Many  sensory  nerves,  however,  terminate  in  special  endings  of 


Termination  of  sensory  nerve  fibres ;  portion  of  the 
plexuses  occupying  the  anterior  layers  of  the  cornea  ; 
gold  preparation  :  n,  n,  nodal  points  of  the  coarser 
ground-plexus  ;  b,  small  bundle  of  nerve-fibrils  which 
breaks  up  into  the  terminal  arbor  (f)  of  ultimate 
fibrillae  ;  v,  fibrils  showing  varicosities. 


g  NORMAL   HISTOLOGY. 

varying  complexity :  of  such  specialized  structures  over  a  dozen 
forms  have  been  described;  since  a  number  of  these  occur  only 
among  the  lower  vertebrates,  the  more  important  types  alone  will  be 
here  considered. 

The  special  sensory  nerve  endings  may  be  grouped  as— 

i.    Tactile  Cells. 

2.  Tactile  Corpuscles. 

3.  End-Bulbs. 

The  tactile  cells  are  found  within  the  deeper  layers  of  the  epi- 
dermis or  the  adjacent  stratum  of  the  corium,  and  may  be  either 
simple    or   compound ;    the    former   are    oval 
Fig.  100.  nucleated  elements,  5-12  fi  in  size,  and  resemble 

ganglion-cells.  The  centrally-directed  portion  of 
the  cells  is  embraced  by  a  peculiar  crescentic 
expansion — the  tactile  meniscus— with  which 
the  nerve-fibre  is  probably  connected. 

Where  two  or  more  such  cells  are  associated  to 
receive  the  nerve-fibre,  a  compound  tactile  cell 
results ;  the  corpuscles  of  Grandry  and  of  Merkel, 
found  respectively  in  the  epidermis  of  birds  and 
of  mammals,  are  examples  of  such  structures. 
The  medullated  nerve-fibre,  on  meeting  the  cells, 
loses  its  neurilemma  and  Henle's  sheath,  these 
coverings  becoming  fused  with  the  connective- 
tissue  capsule  of  the  corpuscle  ;  the  axis-cylinder 
passes  between  the  cells,  to  become  lost  within  an  intercellular 
flattened  tactile  disk;  the  medullary  substance  terminates  at  the 

point  where  the  axis-cylinder 
Fig.  ioi.  enters  the  disk. 

.:.:..-.-..•>,  ....   -,  The   dark    stellate   figures 

-..   r':'--.  ,;.:--  .     .—  .7  J       sometimes  seen  in  gold  prep- 
■".- " '^[j':J>/i       arations    of    the    epidermis, 
lying  between  the  epithelial 
cells,  and  known  as  the  cells 
of  Langerhans,  do  not  rep- 
resent nerve-endings,  as  for- 
merly claimed,  but  are  prob- 
ably migrated  wandering  cells. 
The   more   elaborately  ar- 
ranged compound  tactile  cells 
and  the  simpler  tactile  cor- 
puscles, such  as  the  spherical 
end-bulbs  of  the  conjunctiva, 
are  closely  related,  their  differences  being  but  slight;    the  various 


Termination  of  sen- 
sory nerve-fibres  within 
the  epidermis  ;  gold  prep- 
aration :  e,  deeper  layers 
of  epidermis ;  c,  subja- 
cent connective  tissue  ; 
n,  nerve-fibrilla  pene- 
trating among  the  epi- 
thelial cells. 


Special  nerve-endings  within  the  epidermis ;  gold 
preparation  :  N,  nerve-fibre  entering  the  epithelium 
and  dividing  into  the  fibrils  which  are  connected  with 
the  tactile  disks  (>») ;  upon  these  latter  rest  the  tactile 
cells,  c.     (After  Ranvier.) 


THE    PERIPHERAL    NERVE-ENDINGS. 


85 


Fig.  102. 


Tactile  corpuscles  from  the  bill  of  duck : 
A  ,  simple,  B,  compound,  corpuscle  ;  t,  tactile 
cells ;  d,  tactile  disks ;  n,  medullated  nerve- 
fibres  entering  the  nucleated  capsules  (s)  into 
which  the  neurilemma  continues. 


S33I5 


um 


Fig.  103. 

»:.'•" j  j .X'.'  7>'~; 


tactile  corpuscles  present  increasing  degrees  of  complexity  of  struct- 
ure, the  most  highly  specialized  ending  of  this  class  being  the  tactile 
corpuscle  of  Meissner,  found  in  the 
skin  of  the  palmar  surfaces  of  the 
fingers  and  of  the  toes. 

The  corpuscles  of  Meissner 
are  oval  elliptical  bodies,  45-140  fi 
long  and  35-55  /-*  wide,  situated 
usually  at  the  apices  of  the  papillae 
of  the  corium ;  they  possess  nu- 
merous transversely-placed  nuclei, 
which,  with  the  edges  of  the  indis- 
tinctly defined  tactile  cells,  produce 
the  characteristic  transverse  or  some- 
what spiral  markings.  Each  cor- 
puscle is  supplied  with  one  or  two, 

sometimes  three  or  four,  medullated  nerve-fibres,  which  are  invested 
with  Henle's  sheaths;  the  fibres  may  undergo  numerous  windings 
before  entering  the  corpuscle,  the  sheath  of  Henle,  together  with 
the  neurilemma,  becoming  continuous  with  the 
fibrous  envelope  of  the  corpuscle.  The  nerve- 
fibres  retain  their  medullary  substance  for  a  short 
distance,  but  later  lose  this  sheath  and  break  up 
into  a  number  of  non-medullated  fibres  ;  these 
latter  subdivide  into  fibrillae,  which  pursue  a  spiral 
course  throughout  the  corpuscle,  being  connected 
here  and  there  with  terminal  disks.  The  com- 
pressed tactile  cells  themselves  are  usually  indis- 
tinctly defined,  the  transversely-placed  nuclei  and 
the  outlines  of  the  cells  producing  the  transverse 
markings.  A  large  number  of  the  nuclei  seen, 
however,  belong  to  the  superficial  layers  con- 
tributed by  the  connective-tissue  coverings.  As 
to  the  exact  course  and  mode  of  termination  of 
the  nerve-fibrillae  within  these  tactile  corpuscles, 
much  uncertainty  still  exists. 

The  spherical  end-bulbs  of  the  conjunctiva 
and  of  other  mucous  membranes,  as  well  as  the 
genital  and  the  articular  nerve-corpuscles, 
must  be  included  in  this  class  of  nerve-endings ; 
these  bodies  are  all  formed  on  the  same  general  plan,  the  differences 
in  their  structure  being  limited  to  the  details  of  arrangement. 

The  End- Bulbs.     The  third  group  of  special  nerve-terminations 
embraces  the  nerve-endings  of  a  cylindrical  type  in  contrast  to  the 


m  nn 

Tactile  corpuscle  of 
Meissner  from  the  skin  of 
human  toe  :  JV,  the  nerve 
entering  the  complicated 
group  of  tactile  cells  com- 
posing the  corpuscle ;  Bl, 
blood-vessel  accompany- 
ing the  nerve-fibre.  (After 
Schieffer  decker.) 


86 


NORMAL    HISTOLOGY. 


Fig.  104. 


Fig.  105. 


Genital  corpuscle  from  the 
human  clitoris ;  this  ending 
represents  a  group  of  partly 
fused  simple  spherical  end- 
bulbs  :  n,  nerve-fibres  entering 
the  capsule.     (After  Krause.) 


spheroidal  form  of  those  already  considered.  Just  as  in  the  preceding 
group,  so  here  the  simpler  endings  lead  from  the  tactile  cells  to  the 
more  highly  specialized  structures  ;  the  cylindrical  end-bulbs  of  the 
conjunctiva  of  the  calf  are  the  simplest  members 
of  this  group,  while  the  corpuscles  of  Vater,  or 
the  Pacinian  bodies,  are  its  most  highly  special- 
ized representatives. 

The  nerve-endings  of  this  class  are  composed 
of  three  parts — the  cap- 
sule, the  inner  bulb, 
and  the  nerve-fibre. 
Upon  the  arrangement 
and  development  of  these 
the  differences  distin- 
guishing the  individual 
endings  chiefly  depend. 

In    the   simpler   forms 
of   end-bulbs,     as    those 
found  in  the  conjunctiva 
and     the     oral     mucous 
membrane      of      certain 
mammals,    the    body    is 
borne  upon  a  stalk,  which 
contains  the  medullated  nerve-fibre  and,  possibly,  a  minute  blood- 
vessel enveloped  in  connective  tissue.     The  sheath  of  Henle  invest- 
ing the   nerve  is  prolonged    into 
the  nucleated  capsule.     The  latter 
encloses  a  conspicuous  cylindrical 
mass  of  granular  or  faintly  striated 
pale  substance — the  inner  bulb  — 
within  which  the  free  axis-cylinder 
lies,  terminating  often  in  a  slightly 
marked  knob-like  expansion.    The 
medullary  substance    ends  where 
the  nerve-fibre  enters  the  inner  bulb. 
Further  complexity  in  the  struct- 
ure of  the    end-bulbs    is    largely 
due  to  elaboration  of  the  capsule ; 
this  latter  becomes  laminated  and 
very  thick,  while  the  inner  bulb 
likewise   exhibits    new   details    of 
structure.     Since  the  intermediate 
do  not  occur  in  man,  the  highly 
the  Pacinian  bodies,  may  at 


Simple  spherical  end- 
bulb  from  the  human  con- 
junctival mucous  mem- 
brane :  «,  the  medullated 
nerve-fibre  which  disap- 
pears within  the  capsule. 
(After  Krause.) 


Fig.  106. 


Simple  cylindrical  end-bulbs  from  the  scleral 
conjunctiva  of  calf :  n,  nerve-fibre  passing  into 
the  inner  bulb  {!>) ;  K,  neurilemma  which,  with 
perineurial  sheath  (C),  continues  as  the  capsule, 
C.     (After  Schiefferdecker.) 


forms  in  the  series  of  end-bulbs 
specialized  corpuscles  of  Vater,  or 


THE    PERIPHERAL    NERVE-ENDINGS.  gy 

once  be  considered.  These  structures,  widely  distributed  in  man 
and  mammals,  are  elliptical,  semi-transparent  bodies,  some  2-3  mm. 
long  and  half  as  broad,  which  occur  along  the  nerves  supplying  the 
skin,  especially  of  the  hands  and  feet,  the  external  genitalia,  the 
joints  of  the  extremities,  the  periosteum  of  certain  bones,  the  peri- 
toneum, and  many  other  localities.  Of  the  three  component  parts 
of  the  typical  end-bulb,  the  capsule  has  undergone  the  greatest 
development  in  the  corpuscles  of  Vater,  being  composed  of  25-50 
concentric  connective-tissue  lamellae,  each  of  which  possesses  an 
outer  transverse  and  an  inner  longitudinal  layer  of  fibres,  and  is  lined 

by  a  single  layer  of  endothelial 
Fig.  107.  cells ;   the  nuclei  of  these  plates 

are  seen  in  profile  throughout  the 
capsule.     The  individual  lamellae 

Fig.  108. 


Corpuscle  of  Vater,  or  Pacinian  body,  from  the 
mesentery  of  cat :  N,  nerve-fibre  enclosed  within 
the  perineurial  sheath,  with  which  the  lamellae 
of  the  capsule  {KJ>s)  are  connected ;  K,  nuclei  of 
the  endothelial  plates  of  same ;  Jk,  inner  bulb 
enclosing  the  axis-cylinder  {ax),  which  at  Thp  di- 
vides into  the  terminal  branches.  (After  Ranvier.) 


Herbst's  corpuscle  from  the  bill  of  duck  :  m, 
medullated  nerve-fibre  passing  into  the  interior 
of  the  capsule,  where  the  axis-cylinder  lies  within 
the  granular  inner  bulb  (?')  surrounded  by  a  row 
of  nuclei;  the  spindle  nuclei  appear  between 
the  outer  and  less  closely  placed  lamellae  of  the 
capsule. 


are  separated  by  a  clear  serous  fluid,  which  is  largest  in  amount  be- 
tween the  peripheral  layers,  since  the  lamellae  immediately  surround- 
ing the  inner  bulb  are  thinner  and  more  closely  placed.  The  lamellae 
of  the  capsule  are  often  united  along  a  longitudinal  area — the  intra- 
capsular ligament — which  corresponds  to  the  course  by  which  the 
nerve-fibre  gains  entrance  to  the  inner  bulb  ;  occasional  trabeculae 


88  NORMAL   HISTOLOGY. 

also  pass  between  the  adjacent  lamellae.  After  silver  stainings  the 
corpuscles  of  Vater  appear  to  be  completely  invested  with  a  mosaic 
of  endothelial  plates ;  these  markings  are  due  to  the  cells  which  line 
the  inner  surface  of  the  outer  lamellae. 

The  core  of  the  corpuscle  is  occupied  by  a  light  granular  or  faintly 
striated  cylindrical  mass — the  inner  bulb — composed,  seemingly,  of 
an  almost  homogeneous  tissue,  closely  resembling  protoplasm,  in 
which  nuclei  and  indistinct  fibrils  sometimes  are  seen.  Within  and 
corresponding  to  the  axis  of  the  inner  bulb  lies  the  free  axis- 
cylinder,  ending  frequently  in  a  slightly  expanded  terminal  knob  ; 
the  medullary  substance  surrounds  the  axis-cylinder  as  far  as  the 
inner  bulb,  where  it  disappears.  The  small  artery  usually  accompany- 
ing the  nerve-fibre  within  the  stalk  of  the  corpuscle  gives  off  fine 
branches  to  be  distributed  to  the  outer  layers  of  the  capsule. 

The  corpuscles  of  Herbst,  found  in  birds,  closely  resemble  the 
Vaterian  corpuscles  of  mammals,  possessing,  however,  a  less  devel- 
oped capsule  and  an  inner  bulb  beset  with  a  single  or  double  row 
of  nuclei. 

From  the  foregoing  sketch  it  will  be  seen  that,  taking  the  tactile 
cells  as  a  ground-form,  the  special  nerve-endings  are  developed  along 
two  lines :  one  type  is  represented  by  the  spherical  tactile  cor- 
puscle, composed  of  winding  nerve-fibres  bearing  tactile  disks 
placed  between  tactile  cells  and  enveloped  within  a  capsule  ;  the 
other  by  the  cylindrical  end-bulb,  in  which  the  central  nerve- 
fibre  lies  within  a  cylindrical  inner  bulb,  enveloped  by  a  capsule 
developed  to  a  greater  or  less  degree.  As  the  highest  representative 
of  the  first  group  stand  the  complex  tactile  bodies  of  Meissner ;  of 
the  second  group,  the  corpuscles  of  Vater. 

The  following  table  indicates  the  relations  of  some  of  the  principal 
forms  of  special  sensory  nerve-endings  ; 

Simple  Tactile  Cells. 

(  Ground- Form. ) 
Epidermis  of  man  and  mammals. 

Compound  Tactile  Cells. 

Grandry' s  Corpuscles :  Epidermis  of  birds. 
MerkeV  s  Corpuscles :  Epidermis  of  mammals. 

Tactile  Corpuscles. 
(Spherical.} 

Spherical  End- Bulbs :   Conjunctiva  and  mucous   membranes  of 


89 


THE   PERIPHERAL   NERVE-ENDINGS. 

Ley 'dig '  s  Corpuscles  :  Skin  of  amphibians  and  reptiles. 
Genital  Corpuscles :  Clitoris,  penis,  etc.,  of  man,  etc. 
Articular  Corpuscles :  Phalangeal  joints  of  man,  etc. 
Tactile  End- Bulbs :  Skin  of  bill,  lip,  etc. ,  of  birds. 
Meissner  s  Corpuscles :  Cutis  of  hands,  toes,  etc.,  of  man. 


End-Bulbs. 
(  Cyli?idrical. ) 

Cyli?idrical  End-Bulbs :  Conjunctiva  and  mucous  membranes  of 
mammals. 

End- Capsules :  Buccal  glands  of  hedgehog;  tongue  of  elephant. 

Herbsf  s  Corpuscles :  Skin  and  mucous  membranes  of  birds. 

Key-Retzius  Corpuscles  :  Skin  of  bill  of  birds. 
Vater1  s  Corpuscles  :  Cutis  and  many  other  situations  in  man  and 
mammals. 


XERVE-EXDIXGS    IX   MUSCLE    AXD    OTHER    ORGAXS. 

Non-Striated  Muscle.  The  sympathetic  nerves  supplying  this 
tissue  are  composed  of  bundles  of  non-medullated  nucleated  fibres, 
and  are  enveloped  by  a  thin  perineurium  ;  these  fibres  are  associated 
as  small  bundles  and  unite  to  form  the  ground-plexus,  in  the  nodal 
points  of  which  ganglion-cells  usually  occur.     From  this  net-work 

Fig.  109. 


Nerves  of  involuntary  muscle  from  the  plexus  of  Auerbach  of  intestine  of 
dog;  gold  preparation  :  g,  nodal  points  of  plexus  containing  ganglion-cells  ;  n, 
bundles  of  non-medullated  nerve-fibres  ;  from  these  the  small  branches  {/)  extend 
which  give  off  the  fibres  directly  supplying  the  muscular  tissue. 

small  branches  are  given  off,  which  join  to  make  up  the  intermediate 
plexus  ;  fine  bundles  of  intramuscular  fibrillae  further  extend 
directly  to  the  contractile  tissue.     The  fibrillae  pass  between  the 


9o 


NORMAL   HISTOLOGY. 


primary  bundles  of  the  muscle-cells,  and  probably  terminate  in  finely 
pointed  or  slightly  thickened  free  ends  ;  the  direct  connection  be- 
tween the  nerve-fibrillae  and  the  nuclei  of  the  muscle-cells  is,  at  best, 
extremely  doubtful. 

Striated  Muscle  is  supplied  with  both  motor  and  sensory  nerves  ; 

the  latter  are  distributed 

Fig.  no. 

n 


& 


rri 


Nerves  of  voluntary  muscle  of  rabbit ;  gold  preparation  : 
n,  small  bundle  of  medullated  motor  nerve-fibres,  from  which 
fibres  pass  to  the  individual  muscle-fibres  {m)  and  bear  the 
motor  end-plates  ;  s,  some  of  the  sensory  nerve-fibres  sup- 
plying the  muscle. 


as  a  loose  net-work,  the 
fibrillae  of  which  appar- 
ently terminate  between 
the  individual  muscle- 
fibres. 

The  medullated  nerve- 
fibres  composing  the 
motor  supply  of  a  vol- 
untary muscle  unite  to 
form  an  intramuscular 
plexus,  from  which  small 
bundles  of  nerve-fibres 
spring,  and  subsequently 
divide  in  such  manner 
that  a  single  medullated 
axis-cylinder  passes  to  each  muscle-fibre.  At  the  point  where  the 
nerve   pierces   the   sarcolemma  the   medullary  substance  abruptly 

ends,  while  the  neurilemma,  blended 
with  the  sarcolemma,  joins  the  peri- 
neurial  (Henle's)  sheath  in  forming  the 
telolemma,  or  the  sheath  investing 
the  end-organ.  The  axis-cylinder,  now 
beneath  the  muscle-sheath,  continues 
upon  the  surface  of  the  sarcous  sub- 
stance, and,  later,  breaks  up  into  a 
number  of  somewhat  tortuous  ultimate 
fibrillae,  which  irregularly  unite  and 
end  in  thickened  bulbous  extremities. 
The  terminations  of  the  nerve  are 
embedded  in  a  flattened  nucleated 
mass — the  sole-plate — composed  of 
soft  faintly  granular  protoplasm,  which 
resembles  sarcoplasm  and  is  closely 
applied  to  the  surface  of  the  muscular 
substance;  this  mass,  together  with  the  embedded  nerve-fibrillae, 
constitutes  the  motor  disk,  or  end-plate. 

Each  muscle-fibre  possesses  usually  but  a  single  end-plate ;    in 
exceptional  cases,  however,  there  may  be  two  or  more ;  likewise, 


■m 


Motor  end-plate  of  voluntary  muscle 
from  rabbit:  «,  medullated  nerve-fibre 
passing  to  muscle  (m^,  on  the  surface  of 
which  the  axis-cylinder  ends  in  the  dark 
arborescent  figure ;  the  latter  lies  em- 
bedded within  the  nucleated  sole-plate 
(s)  composed  of  granular  protoplasm. 


THE   PERIPHERAL   NERVE-ENDINGS. 


91 


several  nerve-fibres  instead 
of  a  single  one  may  supply 
the  end-plate. 

The  nerve-endings  in 
the  voluntary  muscle  of 
amphibians  and  bony  fishes 
differ  from  the  foregoing  in 
the  absence  of  the  granular 
protoplasmic  disk,  and  in 
the  more  diffuse  disposi- 
tion of  the  terminal  nerve- 
fibres.  The  axis-cylinders, 
in  these  cases,  branch  into 
fibrillae  which  extend  for 
some  distance  parallel  to 
the  axis  of  the  muscle- 
fibre  and  end  in  slight 
bulbous  expansions  ;  gran- 
ular pyriform  nuclei  also 
occur  along  the  course  of 
these  fibrillae. 

The  muscle-spindles 
described  by  Kiihne,  and 
considered  by  some 
(Kerschner)  as  special 
sensory  nerve  endings, 
appear  to  be  transient 
developmental  structures 
connected  with  the  cleav- 
age of  the  muscle-fibres 
(Kolliker). 

Tendon.  In  addition 
to  the  sensory  end- 
plates  of  tendon,  studied 
by  Kolliker,  Rollett, 
Sachs,  Golgi,  and  others, 
which  consist  of  an  intri- 
cate net-work  of  pale 
non  -  medullated  fibres, 
Golgi  has  described  pe- 
culiar nerve-endings  in 
tendon  to  be  found  in  the 
immediate  vicinity  of  the 
union    with    the    muscle. 


Fig.  112. 


Fs 


II 


mWm 
wSm 


^ 


Ev 


Ev 


Golgi's  corpuscle  or  tendon-spindle  from  the  human  tendo 
Achillis  ;  gold  preparation  :  N,  nerve-fibres  surrounded  by 
the  perineurial  sheath  (Fs)  spreading  out  into  the  reticular 
ramifications  (Ev)  of  the  axis-cylinder;  A,  the  tendon- 
bundles,  one  of  which  is  separated  at  b ;  Jiff,  the  muscle- 
fibres;  R,  node  of  Ranvier.     (After  Ciaccio.) 


Q2  NORMAL   HISTOLOGY. 

These  tendon-spindles  appear  as  sharply-defined,  greatly-elon- 
gated, elliptical  masses  (in  the  rabbit  .25-75  mm.  long  and  .02-. 01 
mm.  broad),  one  end  of  which  extends  upon  the  tendon,  while  the 
muscular  pole  is  usually,  although  not  always,  continuous  with  the 
adjoining  muscle-fibres.  The  tendon-spindle  is  composed  of  a 
distinct  connective-tissue  capsule,  which,  embracing  two  or  more  of 
the  primary  bundles  of  the  tendon,  becomes  united  with  the  sheath 
of  the  latter ;  the  inner  surface  of  the  spindle  is  covered  with  endo- 
thelial plates.  Medullated  nerve-fibres  to  the  number  of  two,  three, 
or  four  join  the  organ  near  its  widest  part,  sometimes,  however,  at 
one  end ;  after  repeated  division  as  medullated  fibres,  the  nerves 
spread  out  on  the  surface  of  the  tendon  as  pale  non-medullated 
fibres,  whose  axis-cylinders  unite  to  form  a  richly  but  irregularly 
meshed  arborescent  figure  ;  the  ultimate  fibrillse,  in  addition  to  the 
net-work,  present  numerous  knobbed  free  ends. 

Blood-Vessels.  The  blood-vessels  are  accompanied  by  nerve- 
fibres  derived  from  the  sympathetic  system  ;  in  addition  to  the  pale 
fibres,  a  few  medullated  ones  usually  take  part  in  the  production  of 

the  irregular  net-work  surrounding 
Fig.  113.  the  larger  vessels.     From  this  plexus 

fine  branches  are  given  off,  which 
ultimately  end  between  the  muscu- 
lar bundles  of  the  media  and  within 
the  fibro- elastic  tissue  of  the  adven- 
titia.      The   capillaries   are   accom- 

Nerve-fibres  accompanying  a  small  artery       panied     and     partly    SUrrOUnded     by 
(v),  from  the  mesentery  of  rabbit ;  gold  prep-         ,  i    11  i 

aration.  delicate      non-medullated       nerve- 

fibres. 

The  muscular  tunics  of  the  large  lymphatic  trunks  are  supplied 
with  nerves  in  a  manner  similar  to  the  blood-vessels  ;  the  delicate, 
thin-walled  lymphatics  are  probably  without  nerves. 

Glands.  A  detailed  account  of  the  nervous  supply  of  the  larger 
glands  will  be  given  in  connection  with  the  consideration  of  the 
several  organs  ;  it  may  be  mentioned  here,  in  general,  that  the  more 
important  glands  are  provided,  in  addition  to  the  medullated  nerves 
often  found  passing  through  the  substance  of  the  gland  in  their  course 
to  the  contiguous  skin  or  mucous  membrane,  with  nervous  bundles 
in  which  non-medullated  fibres  predominate,  but  in  which  some 
medullated  ones  also  occur.  These  bundles  form  an  interlobular 
plexus,  rich  in  ganglion-cells,  which  accompanies  the  larger  excretory 
ducts  and  blood-vessels,  and  gives  off  a  few  branches  to  be  distributed 
to  the  muscular  coats  of  these  tubes.  Thin  bundles  of  pale  fibres 
bear  the  smaller  ducts  company  as  far  as  the  primary  groups  of  acini, 
and  there  break  up  into  minute  bundles  of  free  axis-cylinders  passing 


THE   PERIPHERAL   NERVE-ENDINGS. 


93 


Fig.  114. 


between  the  acini.  The  nerve-fibrillae  may  be  traced  readily  to  the 
membrana  propria  of  the  acini,  around  which  a  net-work  is  spun  ; 
regarding  their  ultimate  distribution  and  relation  to  the  secreting 
cells  much  uncertainty  still  exists,  notwithstanding  many  elaborate 
investigations  and  positive  statements. 

The  exact  mode  in  which  the  nerves  terminate  within  the  acini  is 
still  doubtful ;  it  is  probable,  however,  that  the  fibrillae  end  between, 
or  in  apposition  with,  the  ends  of  the  secreting  cells  directed  towards 
the  basement-membrane ;  proof  of  direct  con- 
nection between  the  nerve-fibrillae  and  the  se- 
creting cells,  as  often  described,  is  wanting. 
Likewise  the  mode  of  termination  of  the  med- 
ullated  fibres,  which,  as  already  stated,  con- 
tribute to  form  the  interlobular  net-work,  is 
uncertain ;  in  some  glands,  as  in  the  pancreas 
of  the  cat  and  the  buccal  glands  of  the  hedge- 
hog, they  terminate  in  special  nerve-endings 
resembling  the  corpuscles  of  Vater. 

The  perceptive  apparatus  connected  with  the 
termination  of  the  nerves  of  special  sense 
include  the  highly  specialized  epithelial  struct- 
ures made  up  of  the  neuro-epithelium  ;  the 
rod-  and  cone-cells  of  the  retina,  the  hair-cells 
of  the  internal  ear,  the  olfactory  cells  of  the  nasal  fossae,  and  the  gus- 
tatory cells  of  the  taste-buds  are  important  examples  of  such  tissue. 
In  these  structures  the  specialized  epithelium  forms  the  apparatus  for 
the  reception  of  the  external  stimuli,  while  the  nerve-fibres  provide 
for  the  further  transmission  of  the  impressions  so  appreciated.  The 
relation  between  the  receptive  cells  and  the  conducting  nerve-fibres 
must  be,  evidently,  very  intimate;  a  direct  anatomical  continuity 
between  the  two,  however,  must  be  regarded  as  extremely  doubtful 
in  the  light  of  recent  research. 


Nerves  ending  in  glands, 
from  the  parotid  of  dog; 
gold  preparation  :  s,  group 
of  secreting  cells  of  single 
acinus  ;  n,  nerve-fibre  lying 
outside  the  membrana  pro- 
pria and  giving  off  twigs 
which  enclose  the  acinus 
within  a  net-work  of  ter- 
minal nerve-fibrilla?. 


94 


NORMAL   HISTOLOGY. 


CHAPTER    VII. 


THE   CIRCULATORY   SYSTEM. 


The  circulatory  apparatus  comprises  the  channels  for  the  con- 
veyance of  the  blood-stream,  the  vessels,  and  the  dilated  and  special- 
ized portion  of  the  vascular  tube,  constituting  the  heart,  for  the  pro- 
pulsion of  the  current.  In  development  and  structure  the  several 
parts  of  the  vascular  system  possess  much  in  common,  although 
variations  in  the  details  of  the  walls  of  the  blood-channels  suffice  to 
distinguish  the  different  portions. 


•■ .  --•?;  •:--  ~r~>  :=-"":  ■~-^_- >;- 


THE    BLOOD-VESSELS. 

The  blood-vessels  occur  in  three  forms,  as  arteries,  veins,  and 
capillaries,  the   latter   constituting   an   expanded  system  of  thin- 
walled  tubules,  intimately  related  to  the  organs,  and  especially  de- 
signed to  facilitate  the  interchanges  be- 
Fig.  115.  tween   the  nutritive  current  which  they 

carry  and  the  tissues  through  which  they 
pass. 

The  arteries  possess  three  coats — the 
inner,  or  intima,  the  middle,  or  media, 
and  the  external,  or  adventitia.  Since 
these  coats  vary  in  relative  thickness  and 


Fig.  116. 


Section  of  human  artery  of  medium 
size  :  /,  the  intima,  consisting  Of  the 
endothelium  (e),  the  sub-endothelial 
tissue  (s),  and  the  internal  elastic 
membrane  (_r) ;  M,  the  media,  com- 
posed of  the  involuntary  muscle  and 
the  bundles  of  elastic  tissue  ( y) ;  A , 
the  adventitia,  containing  irregular 
elastic  trabeculae  (2). 

in  details  of  structure  with  the  size  of  the  vessel,  it  is  usual  to  classify 


Endothelium  of  artery  of  frog  :  the  vessel  has  been  treated 
with  silver,  hence  the  boundaries  of  the  endothelial  plates  are 
indicated  by  the  dark  lines  of  stained  cement-substance.  Sev- 
eral pseudo-stomata  appear  as  minute  dark  areas  between  the 
cells. 


Fig.  118. 


THE   CIRCULATORY   SYSTEM.  g? 

arteries  as  small,  medium,  and  large.  The  first  group  includes  the 
terminal  branches  near  transformation  into  capillaries,  the  second, 
all  the  named  arteries  of  the  body,  except  those  which,  as  the  aorta 
or  the  pulmonary  artery,  are  recognized  as  belonging  to  the  third 
group  of  large  arterial  vessels. 

The  inner  coat,  or  intima,  as  seen  in  a  typical  artery  of  medium 
size,  comprises  three  layers :  (a)  an  endothelial  lining,  made  up 
of  long,  lanceolate,  nucleated  plates,  united  by  a  sinuous  line  of 
cement-substance  and  placed  parallel  to  the  axis  of  the  vessel ;  (6)  a 
sub-endothelial  layer  of  delicate  fibrous  connective  tissue,  with 
branched  corpuscles ;  (c)  a  band  of  elastic  tissue — the  internal 
elastic  membrane — which  forms  the  most  prom- 
inent part  of  the  intima,  appearing  in  sections  of 
medium-sized  ar- 
teries as  a  clear,  Fig.  117. 
glistening,  and 
usually  corrugated 
band  separating 
the  tissue  of  the 
inner  coat  from 
that  of  the  media. 
The  sub-endothe- 
lial tissue,  which 
separates  the  en- 
dothelium from 
the  internal  elastic 
membrane,  is  wanting  in  the  smaller  arterioles,  but  appears  in  vessels 
of  medium  size  as  a  longitudinally  disposed  layer,  becoming  more 
conspicuous  with  the  increased  calibre  of  the  artery.  In  tubes  of 
large  diameter,  as  in  the  aorta,  the  sub-endothelial  tissue  appears  as 
a  stratum  composed  of  layers  made  up  of  fibrous  tissue,  elastic  net- 
works, and  flattened  connective-tissue  cells.  Likewise,  the  elastic 
tissue  of  the  intima  increases  in  amount  and  in  complexity,  in  the 
large  arteries  the  broad  elastic  fibres  becoming  fused  together  to 
form  an  almost  continuous  sheet — the  fenestrated  membrane  of 
Henle. 

The  middle  coat,  or  media,  is  the  muscular  tunic,  and  consists 
principally  of  circularly  disposed  bundles  of  non-striated  muscle-cells  ; 
these  elements,  when  isolated,  appear  as  broad,  nucleated,  irregular 
spindle-cells,  presenting  ragged  outlines.  In  many  arteries,  con- 
spicuously the  subclavian,  the  inner  portion  of  the  media  con- 
tains additional  muscle-cells  longitudinally  disposed.  In  the  smaller 
arteries  the  muscular  tissue  constitutes  almost  the  entire  media,  but 
an  insignificant  amount  of  intermuscular  fibrous  connective  tissue 


Portion  of  the  intima  of  the  human  aorta, 
silver  stained  :  the  larger  stellate  figures  are 
the  cell-spaces  in  the  ground-substance  be- 
tween the  elastic  bundles  and  contain  the 
connective-tissue  corpuscles. 


fii# 

it* 

Portion  of  the  elastic 
tissue  of  the  intima  of 
the  human  aorta ;  the 
fibres  are  so  broad  and 
so  closely  grouped  that 
they  constitute  an  elastic 
sheet  —  the  fenestrated 
membrane  of  Henle. 


Fig 


g5  NORMAL   HISTOLOGY. 

being  present ;  with  the  increase  in  the  size  of  the  vessel,  however, 
the  quantity  of  such  tissue  becomes  greater,  in  addition  to  which 
bands  of  elastic  tissue  also  make  their  appearance  between  the 
muscle-bundles. 

In  the  large  vessels  the  fibro-elastic  tissue  forms  a  considerable 
portion  of  the  media ;  in  the  aorta  the  elastic 
tissue  occurs  as  robust  circularly  arranged  bands, 
supplemented  by  oblique  and  longitudinal  tra- 
becular of  similar  nature  ;  these  elastic  fibres, 
together  with  the  accompanying  fibrous  tissue,  , 
constitute  the  predominating  structure,  the 
muscle  being  less  conspicuous  in  places  than 
the  intermuscular  fibro-elastic  strata. 

Owing  to  this  generous  admixture  of  fibrous 
tissue,  the  large  arteries,  while  possessed  abso- 
lutely of  a  greater  amount  of  elastic  tissue,  have 
walls  relatively  less  contractile  than  those  of  the 
smaller  arteries,  whose  media  is  composed  of 
almost  pure  muscular  tissue. 
The  external  coat,  or  adventitia,  is  the  most  resistant  tunic  of 
the  vessel,   its  characteristic  strength   being  due  to  the   generous 

amount  of  component  fibro-elastic  tissue. 
The  fibrous  tissue  is  arranged  as  closely- 
felted  bundles,  irregularly  placed  and 
intermingled  with  longitudinal  bands  of 
elastic  tissue ;  numerous  flattened  con- 
nective-tissue cells  lie  between  the  bundles 
applied  to  the  fibrous  trabeculae.  The 
mesh-work  is  closer  and  the  amount 
of  elastic  tissue  greater  next  the  media 
than  towards  the  outer  surrounding  con- 
nective tissue  into  which  the  adventitia 
insensibly  blends.  In  the  larger  arteries 
the  middle  and  outer  coats  are  separated 
by  a  band  of  condensed  elastic  tissue — 
the  external  elastic  membrane.  Cer- 
tain arteries  present  peculiarities  in 
their  coats  ;  as  examples  of  such  varia- 
tions may  be  noted  the  slight  develop- 
ment of  the  sub-endothelial  tissue  of  the 
intima  of  the  external  iliac,  renal,  mesen- 
teric, and  cceliac  arteries,  the  appear- 
ance of  longitudinal  muscle-cells  within 
the  intima  of  the  aorta,   and   the  presence  of  longitudinally  dis- 


Muscle-cells  isolated   from 
the  media  of  human  artery. 


Fig.  120. 


Section  of  aorta  of  child :  /,  M, 
and  A,  respectively  intima,  media, 
and  adventitia.  The  thick  stratum 
of  sub-endothelial  tissue  and  the  layer 
of  longitudinally  disposed  bundles  of 
muscle  (6)  are  peculiarities  of  the 
inner  coat. 


THE   CIRCULATORY  SYSTEM. 


97 


C 


posed  muscular  tissue  within  the  adventitia  of  other  vessels  (su- 
perior    mesenteric,     splenic,    renal, 
and  iliac  arteries). 

In  passing  from  medium-sized  ar- 
teries towards  smaller  vessels,  the 
coats  become  reduced  in  thickness, 
the  media  being  earliest  affected. 
The  intima  of  the  smallest  ar- 
terioles consists  of  an  endothelial 
layer  alone,  the  middle  coat  in- 
cludes but  a  single  layer  of  muscle- 
cells,  while  the  external  tunic  is  re- 
duced to  a  few  longitudinal  bundles. 
The  vessels  intermediate  between 
small  arteries  and  true  capillaries 
no  longer  possess  a  complete  layer 
of  muscle-cells,  the  media  being 
represented  in  such  arterioles  by 
scattered  groups  of  circularly  placed 
spindle-cells,  forming  an  imperfect 
muscular  sheet,  which  partially  en- 
circles the  vessel.  The  nuclei  of 
these  circular  muscle-cells  are  trans- 
versely placed,  while  those  of  the 
endothelial  plates  are  usually  longi- 
tudinal or  parallel  with  the  axis  of  the  vessel 


A,  small  human  artery,  in  which  the  coats 
are  reduced  each  to  a  single  layer  of  cells ; 
the  media  here  consists  of  only  one  layer  of 
muscle-cells  (?«),  which  are  seen  in  optical 
section  :  i,  intima  ;  a,  adventitia  ;  e,  nuclei 
of  the  endothelial  plates.  B,  an  arteriole 
just  before  becoming  a  capillary  ;  the  vessel 
still  possesses  muscle-cells  (?«),  but  these 
are  now  arranged  as  irregular  groups  C, 
true  capillary  vessel,  consisting  of  only  an 
endothelial  coat,  the  other  tunics  having 
disappeared ;  the  nuclei  are  those  of  the 
endothelial  plates. 


THE   VEINS. 

The  veins  possess  the  same  tunics  as  the  arteries,  but,  in  general, 
are  characterized  by  thinner  walls  and  a  preponderance  of  connective 
over  the  muscular  and  elastic  tissues.  There  is,  further,  less  regu- 
larity and  constancy  in  the  structure  of  the  coats. 

The  inner  layer  of  the  intima  consists  of  a  single  layer  of  endo- 
thelial cells,  rather  broader  and  more  polyhedral  in  form  than  those 
lining  the  arteries,  the  spindle  shape  being  best  marked  in  the 
smaller  veins.  The  subendothelial  tissue  contains  numerous  con- 
nective-tissue corpuscles,  and,  in  the  larger  veins,  is  arranged  in 
distinct  lamellae.  An  inner  elastic  membrane  is  generally  present, 
in  some  cases  taking  the  form  of  a  fenestrated  layer. 

The  media  consists  of  circular  bundles  of  muscle-cells,  associated 
with  lamellae  of  fibro-elastic  tissue  in  the  larger  veins.  This  coat  is 
best  developed  in  the  veins  of  the  inferior  extremities,  less  so  in 
those  of  the  upper  limbs.  The  muscle-tissue  of  the  veins  is  sub- 
ject to  many  variations,  both  in  amount  and  in  arrangement,  that 

7 


Qg  NORMAL    HISTOLOGY. 

of  the  media  is  very  scant  or  altogether  wanting  in  a  number  of 
veins,  including  the  thoracic  part  of  the  vena  cava,  the  internal  and 
external  jugular  veins,  the  veins  of  the  pia  and  dura,  of  the  retina, 
of  bone,  and  of  the  corpora  cavernosa.  Certain  veins  possess  longi- 
tudinal muscular  bundles  in  the  inner  part  of  the  media  ;  such  are 
the  mesenteric,  umbilical,  iliac,  and  femoral. 

The  adventitia,  often  the  thickest  coat  of  the  vein,  consists  oi 
stout  net-works  composed  of  bands  of  fibro-elastic  tissue  ;  in  some 
veins  additional  bundles  of  plain  muscle  occur  within  this  tunic. 

Among  the  venous  trunks  possessing  well- 
Fig.  122.  marked,    longitudinally  arranged   muscu- 

lar  tissue   in   the   external    coat   are   the 
abdominal  cava,   azygos,   hepatic,   portal, 
-  ~''^1        splenic,     axillary,     superior     mesenteric, 
.,— "i->  renal,  spermatic,  and  external  iliac  veins, 

^y^g^--,/  The  veins  of  the  gravid  uterus  contain 
TL^SS^  muscular  tissue  in  all  the  coats,  the  prin- 
L-^I.r^ .         cipal  bundles  running  longitudinally. 

The  valves  with  which  many  veins  are 
provided  consist  of  crescentic  folds  of  the 
inner  tunic  of  the  vessel,  strengthened  by 
additional  fibro-elastic  tissue ;  in  some 
instances  the  muscular  bundles  extend  for 
a  short  distance  into  the  valve.  The  base 
or  the  attached  margin  of  the  valve  is  often 
its  thinnest  part,  the  free  edges  being 
somewhat  thickened.  The  striated  car- 
diac muscular  tissue  is  continued  for  a 
short  distance  in  the  walls  of  those  parts 
of  the  venae  cavas  and  of  the  pulmonary  veins  immediately  adjoining 
the  heart ;  the  explanation  of  this  fact  is  found  in  the  derivation  of 
these  portions  of  the  vessels  from  the  tissues  of  the  primitive  heart- 
tube. 

THE    CAPILLARIES. 

The  capillaries  establish  the  only  communication,  with  few  excep- 
tions, between  the  arteries  and  the  veins,  and,  further,  provide  the 
intimate  anatomical  relation  between  the  nutritive  current  and  the 
tissues  of  the  body  necessary  for  the  maintenance  of  the  integrity 
and  functional  activity  of  the  various  organs.  As  exceptions  to  the 
usual  intervention  of  the  capillaries  between  the  arterial  and  venous 
radicles,  the  direct  communication  between  these  vessels  existing  in 
the  erectile  tissue  of  the  genital  organs,  in  the  spleen,  and  in  some 
parts  of  the  peripheral  circulation,  as  in  the  tips  of  the  fingers  and 
toes  and  of  the  nose,  may  be  mentioned. 


Section  of  human  vein  of  medium 
size:  /,  M,  and  A,  respectively 
intima,  media,  and  adventitia. 


THE    CIRCULATORY    SYSTEM. 


99 


Fig.  123. 


Capillary  blood-vessels  from  mesentery 
of  young  dog :  ?z,  the  capillaries,  with 
the  nuclei  of  the  endothelial  plates,  lying 
within  the  connective  tissue  ( ' g). 


The  capillaries  form  rich  net-works  in  almost  all  tissues  and  organs, 
the  principal  localities  where  these  vessels  are  wanting  being  epi- 
thelium, the  hairs,  the  nails,  teeth,  cartilage,  the  cornea,  the  crys- 
talline lens,  and  certain  parts  of  the  nervous  system. 

The  capillary  net-works  vary  in  the  size  both  of  the  meshes  and 
of  the  constituent  vessels.  The  average  diameter  of  the  capillaries 
is  7-10  ft ;  the  smallest  are  found  in  the  brain,  retina,  and  muscle ; 
the  largest  in  bone-marrow,  dentinal 
pulp,  and  the  liver.  The  closest  meshes 
are  found  in  the  air- vesicles  of  the  lungs, 
the  choroid,  the  liver,  and  other  glands  ; 
the  widest  in  the  serous  membranes, 
tendon,  etc.  Young  tissues  are  more 
richly  supplied  than  old  ones. 

The  capillaries  consist  of  a  single 
layer  of  endothelial  cells,  united  by 
intercellular  cement-substance ;  they 
are,  consequently,  protoplasmic  tubes 
of  high  vitality,  admirably  designed  to 
facilitate  the  interchanges  constituting 

nutrition.  After  staining  with  silver  the  endothelial  plates  are  seen 
as  extended  spindle-cells,  united  by  irregular  lines  of  darkened 
cement-substance ;  at  the  points  where  the  vessels  branch,  irregular 
triangular  cells  are  not  infrequently  seen.  In  such  preparations, 
likewise,  along  the  lines  of  union  or  at  the  juncture  of  several  plates, 
irregular  darkened  areas — the  stigmata — may  be  observed ;  these 
are  probably  minute  spaces  occupied  by  stained  albuminous  sub- 
stances ;  these  areas  are  supposed  to  aid  the  diapedesis  or  trans- 
migration of  the  blood-cells. 

Some  capillaries  are  invested  by  an  imperfect  adventitious  coat, 
formed  by  a  net-work  of  surrounding  branched  connective-tissue 
cells,  and  resembling  the  reticulum  present  in  lymphoid  tissue.  The 
intimate  relation  existing  between  the  endothelium  of  the  vessels 
and  the  surrounding  connective-tissue  corpuscles  is  well  exhibited  in 
young  growing  tissues,  as  the  omentum. 

The  peculiarities  distinguishing  the  capillaries  from  the  small 
' '  capillary' '  arteries  or  veins  consist  not  so  much  in  the  size  of  the 
vessels — for  the  capillaries  may  have  absolutely  the  greater  calibre — 
as  in  the  character  of  their  walls.  The  true  capillary  possesses  no 
muscle-cells ;  these  first  appearing  in  irregular  groups  beyond  the 
limits  of  the  capillary  vessel ;  in  those  cases  where,  as  in  certain 
veins,  muscular  tissue  is  wanting,  the  character  of  the  adventitia  of 
the  vein  will  aid  in  determining  the  character  of  the  vessel. 

Small  blood-vessels — the  vasa  vasorum — provide  for  the  nutri- 


IOO 


NORMAL   HISTOLOGY. 


tion  of  the  walls  of  the  medium-  and  large-sized  arteries  and  veins. 
These  vessels  arise  some  distance  from  the  area  which  they  supply, 
frequently  coming  from  a  different  branch  or,  as  in  the  case  of  the 
veins,  from  a  neighboring  arterial  stem. 

The  nerves  of  blood-vessels  are  mainly  derived  from  the  sym- 
pathetic system,  and  hence  are  principally  of  the  non-medullated 
kind  ;  a  few  medullated  fibres,  however,  are  usually  present.  The 
nerves  accompanying  the  blood-vessel  give  off  branches,  which  form 
surrounding  plexuses  ;  from  these  minute  bundles  pass,  whose  com- 
ponent fibrillar  are  distributed  to  the  media  and  the  adventitia.  The 
capillaries  are  accompanied  by  correspondingly  delicate  fibres. 

Lymphatic  clefts  and  vessels  are  found  in  the  external  coat  of 
the  larger  vessels.  In  many  places,  as  in  the  nerve-centres,  including 
the  organs  of  special  sense,  in  the  peritoneum,  etc.,  the  lymphatic 
clefts  of  the  adventitia  unite  to  form  a  large  ensheathing  circular 
sinus — the  perivascular  lymph-space — which  separates  a  portion 
of  the  adventitia  from  the  remainder  of  the  vessel ;  as  a  result  of  this 
arrangement,  the  blood-vessel  seemingly  lies  within  the  lymph-space. 
Perivascular  lymphatics  may  be  readily  observed  in  the  peritoneum 
of  the  frog. 

THE    HEART. 

The  heart-walls  consist  of  three  layers — the  endocardium,  the 
muscular  layer,  and  the  pericardium. 

The  endocardium  forms  the  serous  lining 
of  all  parts  of  the  organ,  becoming  continuous 
with  the  inner  tunic  of  the  blood-vessels  at  the 
several  cardiac  orifices.  The  inner  free  sur- 
face of  the  heart  is  covered  with  a  single  layer 
of  polyhedral  nucleated  endothelial  cells. 
These  latter  rest  upon  the  substance  proper 
of  the  endocardium,  a  stratum  composed  of 
fibrous  connective  tissue  mingled  with  a  felt- 
work  of  elastic  fibres ;  the  elastic  net- works 
are  especially  well  developed  in  the  auricles, 
in  certain  parts  of  which  the  broad  fibres  join 
to  form  fenestrated  membranes.  The  outer 
connective-tissue  layer  of  the  endocardium 
is  continuous  with  the  perimysium  of  the 
muscular  tissue. 

The  heart-valves  are  formed  by  duplica- 

tures  of  the    endocardium    strengthened    by 

bands  of  fibrous  tissue  enclosing  numerous 

elastic  fibres.     The  endocardial  layer  of  the  auricular  side  of  the 


Fig.  124. 

1(«  ■  \ 


• 


a 


4,'t: 


; 


)  ^ 


1 


R 

Section  of  human  heart  show- 
ing endocardium  :  a,  endothe- 
lium ;  b,  subendothelial  con- 
nective-tissue stroma  in  outer 
layer  (c),  containing  net-work 
of  elastic  fibres  (e)  ;  d,  trans- 
versely-cut bundles ;  /,  mus- 
cular tissue. 


THE    CIRCULATORY   SYSTEM. 


IOI 


Fig.  125. 


auriculo-ventricular  valves  is  thicker  than  that  of  the  ventricular 
surface.  The  roots  or  attached  portions  of  these  valves  possess 
thickenings  —  the  annuli  fibrosi  —  composed  of  supplementary- 
masses  of  fibro-elastic  tissue.  The  auricular  muscle  is  continued  into 
the  valves  for  about  one-third  of  their  width,  following  closely  the 
general  contours  of  the  fold.  Within  the  larger  chordee  tendineae 
the  papillary  muscles  extend  for  some  distance,  in  addition  to  which 
isolated  muscle-bundles  are  also  sometimes  present.  The  semilunar 
valves  possess  a  thin 
elastic  layer  on  the 
arterial  surface,  aug- 
mented by  a  thick 
stratum  of  connective 
tissue,  the  bundles  ex- 
tending parallel  with 
the  margin  of  the 
valve;  increased 
strength  is  secured  by 
a  nbro-elastic  nodule, 
or  corpus  Arantii, 
which  occupies  the 
middle  of  each  leaflet. 
Beneath  the  ventric- 
ular endocardium,  in 
many  animals  (deer, 
sheep,  calf,  pig,  horse, 
goat,  dog,  certain 
birds,  etc.),  but  not  in 
man,  peculiar  bands — 
the  fibres  of  Purkinje 
— occur ;  these  are 
muscular  fibres  whose 
transverse  striations 
are  limited  to  the  pe- 
ripheral zone,  while 
their  centre  is  occupied 
by  a  large  continuous 
mass  of  nucleated  pro- 


Section  of  the  heart,  including  a  leaflet  of  the  semilunar  valve 
of  the  pulmonary  artery  of  child:  a,  a,  cardiac,  b,  b,  arterial, 
surface;  c,  recess  behind  the  valve  {/),  constituting  part  of  a 
sinus  of  Valsalva ;  d,  free  border  of  valve  ;  e,  thickening  near 
edge  of  valve  corresponding  to  a  corpus  Arantii  ;  g,  endothe- 
lium, h,  intima,  i,  media,  k,  adventitia,  of  the  pulmonary  artery  ; 
the  adventitia  is  continuous  with  the  principal  fibrous  layer  of 
the  endocardium  ;  m,  cardiac  muscle  ;  n,  areolar  tissue. 


toplasm.       The    fibres 

of  Purkinje  represent  an  embryonal  condition  of  the  muscular  tissue, 
since  the  peripheral  part  of  the  fibre  alone  has  undergone  differen- 
tiation, while  the  central  portion  has  remained  indifferent  protoplasm. 
Among  some  lower  vertebrates,  as  fishes,  a  similar  condition  of  the 
muscle-fibres  is  constant. 


102 


NORMAL    HISTOLOGY. 


Fig.  126. 
d    c     h  * 


The  muscular  tissue  of  the  heart  possesses  the  peculiarities 
already  described  in  Chapter  IV. :  it  is  composed  of  short,  branched, 
nucleated  fibre-cells,  devoid  of  a  sarcolemma,  which  unite  to  form 
an  intricate  net-work.  The  naked  muscle-fibres  are  enveloped 
within  a  perimysium  and  are  grouped  into  primary  and  secondary 
bundles,  which  are  associated  to  form  lamellae  disposed  in  a  very 
irregular  and  complex  manner. 

The  muscular  tissue  of  the  auricles  is  arranged  in  general  as  an 
outer  transverse  and  an  inner  longitudinal  layer,  many  small  ad- 
ditional bundles  deviating  from  the  principal  disposition  to  pursue 

independent  courses  in  various  directions. 
The  muscle-bundles  of  the  ventricles  have 
a  very  intricate  arrangement,  the  majority 
extending  in  an  irregular  oblique  or  spiral 
direction,  some,  in  fact,  describing  a  figure- 
of-eight  in  their  course. 

The  pericardium,  which  invests  the 
exterior  of  the  heart,  and  by  reflection 
forms  the  pericardial  sac,  resembles  the 
endocardium  in  possessing  a  single  layer 
of  endothelial  plates  covering  its  free 
surface,  and  a  stratum  of  fibro-elastic  con- 
nective tissue  beneath.  The  parietal 
pericardium  is  distinctly  thicker  than  the 
visceral,  all  the  constituent  layers  being 
better  developed.  The  subpericardial 
tissue  covering  the  heart  is  continuous 
with  the  intermuscular  connective  tissue 
of  the  outer  muscular  layer ;  in  this  posi- 
tion numerous  fat-cells  lie  between  the 
bundles  of  the  fibrous  and  the  muscular 
tissue. 

The  blood-vessels  supplying  the  muscle 
of  the  heart  are  derived  as  branches  of 
The  principal  trunks  are  situated  in  the 
larger  interlamellar  masses  of  connective  tissue,  within  which  they 
divide  into  numerous  twigs  giving  origin  to  the  capillaries ;  the 
latter  penetrate  the  primary  muscle-bundles,  among  and  parallel  to 
which  they  run.  The  relation  between  the  individual  muscle-fibres 
and  the  capillaries  is  more  intimate  than  usually  supposed,  since,  as 
shown  by  Meigs,  the  blood-vessels  deeply  impress  the  fibres,  and 
in  many  places  are  surrounded  completely  by  the  muscular  tissue. 
The  extraordinary  demands  made  upon  the  nutrition  of  the  heart- 
tissue  as  the  result  of  its  remarkable  functional  activity  explain  the 


W  -  ■  1 

Section  of  human  heart,  including 
pericardium  :  a,  endothelium  of 
pericardial  surface ;  b,  subendo- 
thelial  fibrous  tissue  ;  c,  net-works 
of  elastic  fibres  ;  d,  subpericardial 
areolar  tissue  containing  fat-cells 
embedded  between  pericardium  and 
muscle  (e)  ;  v,  blood-vessel. 


the 


coronary  arteries. 


THE    CIRCULATORY    SYSTEM. 


IO3 


necessity  for  such  close  arrangement.  The  deeper  fibrous  layers 
of  the  pericardium  and  of  the  endocardium  receive  numerous  capil- 
laries, a  few  being  also  found  within  the  chordae  tendineae  and  the 
valves. 

The  lymphatics  of  the  heart  are  very  numerous.  They  form 
a  comprehensive  system,  embracing  the  lymph-spaces  occupying 
the  clefts  between  the  muscle-fibres  and  the  rich  net-works  of 
more  definite  channels  extending  within  the  pericardium  and  endo- 
cardium, including  the  valves.  These  two  sets  of  lymph-radicles 
communicate  but  sparingly  and  pursue  largely  independent  courses. 
Lymphatic  vessels  also  accompany  the  branches  of  the  coronary 
arteries. 

The  rich  nervous  supply  of  the  heart  is  derived  from  the  coro- 
nary plexuses,  and  includes  numerous  medullated  fibres  coming  from 
the  pneumogastric,  as  well  as  the  non-medullated  sympathetic  fibres 
proceeding  from  the  cervical  ganglia.  Numerous  microscopic  gan- 
glia are  found  along  the  course  of  the  larger  nerve-trunks  accom- 
panying the  branches  of  the  coronary  arteries,  especially  in  the 
longitudinal  interventricular  and  in  the  auriculo-ventricular  furrows. 
Many  additional  small  groups  of  ganglion-cells  occur  within  the 
muscular  tissue  associated  with  the  fibres  supplying  the  intimate 
structure.  The  nerves  and  the  blood-vessels  are  covered  by  the 
visceral  pericardium. 

The  development  of  all  parts  of  the  circulatory  apparatus 
takes  place  within  the  mesoderm ;  while  possessing  a  common 
origin,  the  blood-vessels  and  the  heart,  however,  develop  in- 
dependently, and,  for  a 
time,  are  distinct  and  dis- 
connected. The  earliest 
blood-vessels  appear  near 
the  periphery  of  the  vascu- 
lar area,  outside  the  limits 
of  the  proper  body  of  the 
embryo  ;  later  and  second- 
arily they  extend  centrally 
and  unite  with  the  primitive 
heart  and  those  parts  of  the 
large  trunks  which  have 
been  formed  coincidently 
within  the  embryo. 

The  mesodermic  elements  within  certain  tracts  near  the  periphery 
of  the  vascular  area  undergo  proliferation,  which  results  in  the  pro- 
duction of  deeply  staining  densely  nucleated  areas  known  as  the 
blood-islands  of  Pander ;  these  are  the  direct  progenitors  of  the 


Fig.  127. 


Developing  capillary  blood-vessels  within  the  omentum 
of  young  rabbit :  a,  a,  elongated  protoplasmic  processes 
connecting  the  walls  of  the  newly-formed  capillary  (c) 
with  the  angioblastdc  connective-tissue  corpuscles  (3). 


IQ4  NORMAL    HISTOLOGY. 

earliest  blood-vessels  and  the  first  blood-cells.  The  blood-channels 
appear  within  the  nucleated  "islands"  as  spaces  which  follow  the 
partial  breaking  down  of  the  inner  portions  of  the  areas.  The 
peripheral  zone  of  the  nucleated  cell-mass  becomes  the  endothelium 
of  the  future  blood-vessel,  while,  probably,  certain  of  the  enclosed 
mesodermic  elements  persist  as  the  primary  blood- cells.  After  a 
time  the  mesoderm  surrounding  the  newly-formed  endothelial  tube 
differentiates  into  the  muscular  and  other  tissue  of  the  remaining 
coats.  The  endothelium  is,  therefore,  genetically  the  oldest  part 
of  the  vessel,  although  its  characteristic  appearance,  as  seen  in 
silvered  adult  tissue,  is  not  visible  until  further  differentiation  has 
taken  place. 

The  blood-channels  are  further  extended  by  the  fusion  of  elongated 
mesoblastic  cells  with  those  of  the  walls  of  the  primary  vessels,  the 
lumina  of  the  latter  gradually  entering  the  solid  processes,  which  are 
thus  converted  into  tubes.  After  the  development  of  the  earliest 
vessels  in  the  manner  indicated,  the  formation  of  all  new  vessels 
subsequently,  in  pathological  processes  as  well  as  in  normal  ones, 
is  associated  closely  with  the  connective-tissue  cells,  since 
solid  protoplasmic  processes  of  the  united  cells  become  later  the 
walls  of  the  young  vessel. 

The  development  of  the  heart  resembles  that  of  the  extra- 
embryonic vessels  in  so  far  that  the  part  first  formed— the  primary 
endothelial  tube— originates  by  the  differentiation  of  the  mesodermic 
cells  and  the  hollowing  out  of  the  tissue 
lying  enclosed.      In  its  very  early  stage  the 
•/!>  '  <s^  mammalian  heart  exists  as  two  distinct  and 

(i^i/®^^  widely-separated  tubes,  which  later  unite  to 

|i\V/,6  *     ^M)         form  a  single  sac-      Outside  the  primary 
lh^T®0i^:  endothelial    heart  the  mesoderm  differ- 

■*&&g®pfji<  entiates   into    the    muscular   tissue   of  the 

Section  ofa  part  of  the  develop-  cardiac  wall,  but  for  some  time  the  endo- 
Sv!n.?:^t«tLi  thelial  and  muscular  layers  continue  as 
tube,  within  which  lie  several  of  independent  tubes,  the  inner  endothelial 
tir^:S%^  SS£  "™g  appearing  as  a  shrunken  cast  repro- 
ennated  mesoblastic  cells,  which     ducing  the  contours  of  the  larger  muscular 

later  become  the  muscular  tissue.        Qrgan         The  tWO    tunics    are    Connected    by 

numerous  bridging  bands,  which  increase  in  number  and  size  with 
the  progress  of  the  development  of  the  organ  ;  these  primary  tra- 
becular are  represented  in  the  adult  organ  by  the  columnar  carneae 
and  musculi  pectinati.  The  pericardium  originates  as  the  special- 
ized layer  of  mesoderm — the  mesothelium — forming  the  immediate 
boundary  of  the  general  primary  body-cavity,  of  which  the  peri- 
cardial sac  is  only  a  constricted  portion. 


THE  CIRCULATORY  SYSTEM.  IOf 


THE  BLOOD. 


While,  when  physiologically  considered,  the  blood  is  regarded, 
with  Bernard,  best  as  an  internal  medium  of  exchange,  histologically 
it  may  be  classed  as  a  mesodermic  tissue  possessing  a  fluid  inter- 
cellular substance,  the  liquor  sanguinis  ;  in  the  latter  float  the 
cellular  elements — the  blood-corpuscles. 

The  morphological  constituents  of  the  blood  are  of  two  kinds, 
the  colorless  or  white  corpuscles  and  the  colored  or  red  cells  ; 
to  these  must  be  added  a  third  variety,  the  blood-platelets  or 
blood-plaques,  which  are  probably  constant  and  independent  ele- 
ments. 

THE  COLORLESS  CELLS  OF  THE  BLOOD. 

The  colorless  blood-cells,  or  leucocytes,  are  not  peculiar  to 
the  blood,  since  they  originate  in  lymphoid  tissues  and  are  carried 
by  the  lymphatic  trunks  into  the  blood-current,  in  which  fluid  they 
usually  are  observed.     These  cells  represent 
a   widely-distributed    element,    whose    names  Fig.  129. 

are  as  various  as  are  the  localities  in  which 
it  is  encountered.  The  "lymph-corpuscle," 
"lymphoid  cell,"  "adenoid  cell,"  "white 
blood -cell,"  "leucocyte,"  "  leucoblast, " 
"wandering  cell,"  etc.,  are  but  different  J|||| 
names  for  the  same  morphological  element. 

The  colorless  blood-cell  consists  of  a  minute       Colorless  biood-cdis  of  man, 

.  highly     magnified :     r,     cor- 

nucleated  mass  of  active  protoplasm,  when  at  puscie  in  condition  of  rest, 
rest,  presenting  a  round  or  spherical  form  and     as  asPherical  mass  of  Prot°- 

.  ,       ■  ...  .  plasm ;    the    other    cells    are 

measuring  about  10  fi  in  diameter.  In  its  actively  moving  and  exhibit 
usual  condition,  however,  the  outline  of  the     a  hyaline  apparently  struct- 

.        .  ,  .  j  ureless  substance  in  the  most 

corpuscle  is  undergoing  continual  variation,     advanced  pans  of  the  ceils. 
these  changes  being  known  as  amoeboid  on 

account  of  their  similarity  to  those  exhibited  by  the  amoeba.  Under 
moderate  amplification  the  protoplasm  of  the  leucocyte  appears 
faintly  granular  and  includes  a  single  nucleus,  rarely  multiple,  which 
is  ordinarily  somewhat  obscured  by  the  overlying  cell-contents. 
Additional  coarse  granules  are  of  frequent  occurrence,  especially 
within  the  protoplasm  of  particular  cells ;  these  latter,  sometimes 
designated  as  the  granular  white  corpuscles,  are  distinguished  by 
their  large  granules,  small  size,  and  affinity  for  eosin,  and  are  re- 
garded by  some  as  a  special  variety  of  colorless  blood-cell.  The 
exact  nature  and  significance  of  such  cells,  however,  are  still  un- 
certain. Under  high  amplification  the  protoplasm  of  the  white 
blood-cell   often   displays   an    imperfect  reticulation   as  a  transient 


IQ5  NORMAL   HISTOLOGY. 

structure,  as  well  as  nuclear  fibrils.  Pole-corpuscles  and  at- 
traction-spheres have  been  described  by  Flemming  as  constant 
constituents  of  the  white  blood-corpuscle.  Division  of  these 
elements  in  many  instances  undoubtedly  is  accompanied  by  the 
regular  cycle  of  karyokinesis  ;  very  commonly,  however,  it  is  equally 
certain,  the  colorless  corpuscles  are  reproduced  by  direct,  amitotic 
division.  Examination  of  actively  moving  cells  under  high  amplifi- 
cation emphasizes  a  distinction  in  the  character  of  the  protoplasm, 
that  part  of  the  cell  constituting  its  most  advanced  portion  seem- 
ing more  homogeneous  than  the  remainder  of  the  body  of  the 
cell. 

The  colorless  cells  of  human  blood  are  larger  than  the  red 
corpuscles,  but  are  much  fewer  in  number,  the  ratio  between  the 
two  kinds  of  elements  being,  under  normal  conditions,  about  three 
hundred  and  fifty  red  cells  to  one  white  corpuscle.  The  actual 
number  of  white  cells  present,  however,  depends  upon  various  cir- 
cumstances, since  during  digestion  the  number  of  colorless  elements 
is  increased,  while  fasting  greatly  reduces  the  proportion  of  the 
leucocytes ;  in  general  these  cells  are  more  numerous  in  venous  than 
in  arterial  blood. 

The  colorless  blood-cells  must  be  regarded  as  playing  a  double 
role :  in  addition  to  maintaining  an  ever-available  store  of  reserve 
active  protoplasm  with  which  to  meet  and  to  repair  the  destructive 
processes  taking  place  normally  as  well  as  in  disease,  they  are 
actively  engaged  in  the  absorption  of  solid  and  fatty  matters,  being 
capable  of  taking  up  and  carrying  away  injurious  debris.  Certain 
of  these  cells — the  phagocytes  of  Metschnikoff — seem  especially 
aggressive  in  their  attacks  against  offending  foreign  substances, 
within  a  limited  degree  including  possibly  the  waging  of  a  successful 
warfare  on  obnoxious  microbes. 

THE  COLORED  CELLS  OF  THE  BLOOD. 

The  adult  mammalian  red  blood-cell  represents  a  condition  of 
retrogression,  since  in  its  development  it  has  suffered  the  loss  of 
its  nucleus  and  a  profound  metamorphosis  of  its  protoplasm,  changes 
of  such  importance  that  some  authorities  dispute  the  propriety  of 
regarding  the  mammalian  red  blood-corpuscles  as  true  cells.  The 
presence  or  absence  of  the  nucleus  within  the  colored  corpuscle, 
together  with  its  general  form,  furnishes  a  basis  for  a  division  of  all 
vertebrate  bloods  into  — 

A.  Those  having  nucleated,  oval  red  corpuscles  :  including 
fishes  (except  cyelostomata,  which  have  round,  discoidal  cells,  as  the 
lamprey),  amphibians,  reptiles,  and  birds. 

B.  Those  having  non-nucleated,  round,  discoidal  red  cor- 


THE   CIRCULATORY   SYSTEM, 


IO7 


puscles  :    including   man  and   other  mammals,   except  the  camel 
family,  which  have  oval,  non-nucleated  red  blood-cells. 

Since  an  oval  corpuscle  on  being  subjected  to  certain  reagents 
may  present  a  circular  outline,  the  presence  or  absence  of  a  nucleus 
offers  the  most  reliable  means  of  differential  diagnosis  between 
mammalian  and  other  bloods. 

The  human  colored  blood-cell  is  a  small  round  disk,  measuring 
about  8  [i  in  diameter,  and  exhibiting  individually  a  faint  greenish- 
yellow  tinge.     The  well-known  color  of  the 
blood  appears  only  when  great  numbers  of  FlG- 

these  corpuscles  are  massed  ;  the  term  "  red" 
conventionally  applied  to  these  elements  is, 
strictly  regarded,  incorrect  and  less  appro- 
priate than  "colored."  The  two  surfaces  of 
the  blood-disk  are  not  perfectly  flat,  the  centre 
of  the  corpuscle  being  slightly  biconcave, 
while  its  edges  are  rounded,  biconvex,  and  Human  biood-ceiis :  w,  coior- 
somewhat  thickened  :  in  consequence  of  this     less  corpuscle,  surrounded  by 

,.  ,  .  .   ,,      r  ..         .  r       ,  red  cells;  those  at  r  exhibit  a 

peculiar    "biscuit      form,    all   planes   of  the     partiaiiy-formed rouleau. 
corpuscle  are  not  seen  accurately  focused  at 

one  time,  the  centre  usually  appearing  either  darker  or  lighter  than 
the  marginal  parts  of  the  cell,  depending  upon  the  focal  adjustment. 

The  structure  of  the  colored  blood-corpuscles  is  still  a  subject  of 
discussion.  According  to  the  generally  accepted  view,  the  cor- 
puscles consist  of  two  parts  :  (a)  the  transparent,  colorless,  apparently 
homogeneous,  and  plastic  stroma,  extensible  and  pliable  to  a  high 
degree,  and  (b~)  the  coloring  matter,  or  haemoglobin,  which  is  held 
within,  and  uniformly  distributed  throughout,  the  former.  This 
conception  of  the  corpuscle  assumes  the  presence  of  a  uniform 
though  highly  flexible  stroma-mass  of  definite  form,  colored  by  the 
imbibition  of  the  soluble  haemoglobin.  On  the  other  hand,  the 
behavior  of  these  elements  when  treated  with  water,  upon  the 
addition  of  which  the  corpuscles  swell,  lose  the  discoidal  form,  and 
become  globular,  as  well  as  the  suggestive  appearances  following  the 
staining  with  aniline  of  such  bleached  corpuscles,  the  outlines  of  the 
cells  then  showing  as  distinct  rosy  rings,  offers  strong  arguments,  in 
the  opinion  of  not  a  few,  for  the  belief  that  the  red  corpuscles  are 
minute  sacs,  consisting  of  a  limiting  membrane  and  the  colored  fluid 
contents. 

The  nuclei  of  the  red  cells,  when  present,  lie  embedded  within 
the  colored  stroma  ;  in  perfectly  fresh  or  circulating  corpuscles  they 
are  made  out  with  great  difficulty,  since  they  possess  a  refractive 
index  almost  identical  with  that  of  the  other  parts  of  the  cell.  After 
reagents,  or  after  the  expiration  of  some  minutes,  the  nuclei  become 


108  NORMAL   HISTOLOGY. 

very  evident,  and  correspond  in  appearance  and  structure  with  those 
of  other  cells,  one  or  more  nucleoli  often  being  visible. 

In  fresh  blood  the  red  corpuscles  within  a  few  minutes  arrange 
themselves  in  rows  or  piles  by  the  apposition  of  their  broader  sur- 
faces, thus  forming  figures  which,  from  their  resemblance  to  rolls 
of  coin,  are  termed  rouleaux.  The  cause  of  this  phenomenon  is 
still  uncertain,  although  it  is  not  improbable  that  it  is  to  be  attributed 
to  the  presence,  in  the  fresh  corpuscles,  of  a  film  of  a  nature  repelling 
the  liquor  sanguinis  and  favoring  the  adhesion  of  the  disks  ;  the 
rouleaux  are  only  temporary,  the  corpuscles  later  spontaneously 
separating  and  remaining  apart.  It  is  of  interest  to  note  that  only 
discoidal  corpuscles  of  mammalian  bloods  (including,  however,  the 
discoidal  cells  of  the  lamprey)  run  together  to  form  these  figures, 
the  projecting  nuclei  and  the  slight  biconvexity  of  the  oval  nucleated 
cells  affording  surfaces  evidently  unfavorable  for  adhesion. 

The  average  diameter  of  the  red  corpuscles  in  the  various  races 
of  mankind  is  identical,  being  between  7  and  8  /*,  or  about  1  -3200th 
of  an  inch. 

The  size  of  the  animal  bears  no  relation  to  that  of  its  red  blood- 
cells,  as  shown  by  the  following  measurements  of  some  mammalian 
bloods,  based  on  the  observations  of  Gulliver  : 

Millimetre  Millimetre.  Millimetre. 

Elephant  .    .    .    .0092  Guinea-pig  .    .    .0071  Pig 0060 

Sloth 0086  Dog 0071  Horse 0059 

Whale 0080  Rabbit 0070  Cat 0058 

Man 0079  Bear 0070  Sheep 0048 

Beaver 0076  Mouse 0067  Goat 0040 

Monkey    .    .    .    .0074  Ox 0048  Muskdeer     .    .    .0024 

The  largest  corpuscles  are  those  of  the  amphibians,  the  red  cells  of 
the  frog  measuring  .0016  mm.  in  breadth  by  .022  mm.  in  length,  those 
of  the  triton,  .019  by  .029,  and  those  of  the  proteus,  .035  by  .058. 
The  maximum  size  is  reached  in  the  huge  red  cells  of  the  amphiuma, 
which  are  no  less  than  .046  mm.  wide  by  .075  mm.  long,  and  are 
readily  distinguishable  by  the  unaided  eye. 

The  number  of  colored  cells  normally  present  in  one  cubic 
millimetre  of  human  blood,  as  determined  by  the  hsemacytometer, 
is  about  five  millions  ;  these  figures  are  modified  by  sex,  the  male 
subject  usually  having  more  corpuscles  than  the  female. 

The  number  of  red  corpuscles  varies  in  different  animals  :  the 
carnivora  possess  a  greater  number  of  cells  in  a  given  quantity  of 
blood  than  do  the  herbivora  ;  in  birds  the  proportion  is  still  larger  ; 
while  in  the  sluggish  amphibians  the  number  of  the  huge  red  cells 
is  reduced  to  thousands. 

Effect  of  Reagents  applied  to  Human  Blood.     No  elements 


THE   CIRCULATORY   SYSTEM. 


IO9 


Fig.  131. 


Red  blood-cells  of  man  and  of  am- 
phiuma,  magnified  to  the  same  extent 
to  show  the  size  of  the  human  cor- 
puscles in  comparison  with  that  of  the 
largest  known  blood-cell. 


are  more  sensitive  to  changes  in  environment  or  to  the  effects  of 
reagents  than  are  the  cells  of  the  blood.  An  appreciation  of  the 
alterations  referable  to  external  causes  is  important  as  guarding 
against  unwarranted  conclusions  as  to  the  existence  of  pathological 
conditions,  since  not  infrequently  ap- 
pearances which  lead  the  tyro  to  infer 
disease  may  be  ascribed  to  influences 
acting  on  the  corpuscles  outside  the 
body. 

If  fresh  blood  be  exposed  to  a  current 
of  air,  subjected  to  undue  pressure  or  to 
other  disturbing  influences,  alterations  of 
the  corpuscles  at  once  take  place.  One 
of  the  most  common  distortions  affects 
the  exterior  of  the  red  corpuscles,  and 
results  in  the  formation  of  a  number 
of  minute  projections,  or  spines,  pro- 
ducing a  condition  known  as  crenation. 

Saline  Solutions.  The  application  of  a  weak  saline  solution  or 
of  urine  is  attended  with  similar  effect ;  if  the  strength  of  the  reagent 
be  gradually  increased,  a  corresponding  progressive  degree  in  the 
distortion  is  observed,  until,  finally,  upon  the 
addition  of  a  concentrated  brine,  a  shrivelled, 
shapeless  mass  replaces  the  former  discoidal 
red  corpuscle.  The  reaction  is  less  marked 
upon  the  colorless  cells,  weak  salines  pro- 
ducing no  perceptible  change,  while  a  slight 
shrinkage  is  noticeable  after  the  stronger 
solutions. 

Water.  Upon  the  application  of  water 
the  colored  cells  swell  up,  lose  the  discoidal 
form,  and  become  spherical,  and  at  the  same 
time  part  with  their  coloring  matter,  the 
haemoglobin ;  the  latter,  being  dissolved, 
leaves  the  bleached  and  colorless  stroma  to 
form  the  ' '  ghost. ' '  That  the  red  corpuscles 
are  not  destroyed  by  the  water,  as  sometimes 
stated,  may  be  demonstrated  by  the  addition 
of  a  suitable  aniline  dye,  when  the  presence 
of  the  bleached  corpuscles  is  made  evident  by 
the  colored  rings  which  mark  their  outlines. 

The  action  of  water  upon  the  living  color- 
less blood-cells  is  somewhat  different. 
These   corpuscles   cease   their  amoeboid   movements,   retract   their 


FlG 


Reactions  of  human  blood- 
cells  with  various  reagents : 
A ,  effect  of  treatment  with 
water  upon  the  white  (w)  and 
the  colored  cell  (r) ;  B,  red 
cells  after  the  addition  of  saline 
solutions,  crenation  following 
the  application  of  weak  solu- 
tions, great  shrinking  and  dis- 
tortion (s)  succeeding  the 
action  of  the  concentrated 
reagent ;  C,  action  of  dilute 
acetic  acid  on  the  colorless 
cell  (w)  and  on  the  red  cor- 
puscle (r) ;  D,  red  blood-cell 
after  the  addition  of  one-per- 
cent, solution  of  tannic  acid. 


no 


NORMAL    HISTOLOGY. 


processes,  become  round,  and  swell  up  into  larger  spheres  ;  mean- 
while, the  protoplasm  resolves  itself  into  a  number  of  sharply-cut 
granules,  which,  owing  to  their  suspension  within  a  fluid  of  less 
density  than  the  blood-plasma,  exhibit  the  active  dancing  or  oscil- 
latory movements  which  constitute  the  "  Brownian  motion,"  a 
phenomenon  entirely  physical  in  nature.  The  nuclei  of  the  colorless 
cells  after  treatment  with  water  appear  as  clear  or  slightly  granular 
areas  among  the  vibrating  particles.  After  a  time  the  distention 
of  the  corpuscle  becomes  too  great,  and  rupture  takes  place,  followed 
by  the  escape  of  the  particles  of  disintegrated  protoplasm. 

Acids.  Upon  the  addition  of  weak  acetic  acid  the  red  cells 
become  rapidly  decolorized,  at  the  same  time  losing  the  discoidal 
form  and  approaching  the  spherical.  The  protoplasm  of  the  color- 
less corpuscles  clears  up  entirely,  the  nuclei  coming  very  con- 
spicuously into  view.  Upon  subsequent  treatment  of  acid  prepara- 
tions with  aniline,  the  nuclei  of  the  white  cells  appear  deeply  stained, 
while  the  red  cells  are  outlined  by  faintly-colored  rings. 

Tannic  acid,  when  applied  to  the  red  corpuscles  in  weak  (one- 
half  to  one  per  cent.)  solutions,  produces  a  peculiar  effect:  the 
coloring  matter  of  the  corpuscle  is  coagulated  as  it  escapes  from  the 
cell  and  becomes  conspicuous  as  a  minute  accumulation  adhering  to 
one  edge  of  the  corpuscle.  Where  strong  solutions  of  tannic  acid 
are  employed,  the  haemoglobin  is  coagulated  within  the  corpuscle 
before  it  has  had  an  opportunity  to  escape,  producing  appearances 
which  have  been  mistaken  for  nuclei  and  other  details  of  cell-structure. 

The  Blood-Platelets.     If  human  blood  be  drawn  directly  into 

a  drop  of  osmic  acid  solution  (one  per  cent.)  or  of  a  three-fourths 

per    cent,     solution    of    sodium     chloride, 

Fig.  133.  covered  at  once,  and  examined  with  a  high 

power,  numbers  of  small,  colorless,  circular 

QOO  disks  will  be  seen  on  careful  observation; 


these  are  the  blood-platelets  of  Bizzozero, 
O/*  'W-.       /^>  '  *  sometimes  called  the  third  corpuscular  ele- 

%&9         ]$g\ — "    ments  of  the  blood.     They  are  very  unstable, 

prone  to  disintegration,  and  are  variable  in 
a,  human  red  blood-ceils  and     size,  possessing  an  average  diameter  of  about 

blood-phatelets    (/):  ^minute       Qne.third     of    that     of    the    red     cdl  they 

fatty   (?)   particles,  which   occur  _    J 

isolated  or  in  masses;  .s,  fibrin     occur  singly,  but  show  a  marked  disposition 

SE^",E£E  to  run  to^ether  in  -rouPs  PreParatory to 

lets.  breaking  up  into  the  minute  particles  long 

known  as  the  granules  of  Max  Schultze. 
Unless  great  precaution  is  taken  to  insure  the  immediate  action  of 
the  preserving  fluids,  the  blood-platelets  will  not  be  seen  in  their 
normal  form. 


THE    CIRCULATORY    SYSTEM. 


Ill 


These  bodies  may  be  recognized  in  the  circulating  blood,  as  ob- 
served by  Osier  and  others,  and  are  constant,  although  numerically 
variable,  elements  of  mammalian  bloods.  The  peculiar  elongated 
elliptical  "blood-spindles"  found  in  the  blood  of  other  vertebrates 
are  probably  to  be  regarded  as  the  homologues  of  the  blood-plaques 
of  mammals.  While  the  presence  of  the  blood-platelets  as  distinct, 
constant,  and  normal  constituents  of  the  human  blood  is  now  gener- 
ally recognized,  authorities  are  far  from  accord  as  to  their  significance. 
The  evidence  at  present  seems  to  point  to  a  close  relation  between 
these  bodies  and  the  process  of  coagulation,  in  view  of  their  probable 
active  role  in  the  production  of  the  factors  in  the  formation  of  fibrin. 

Fibrin  filaments  are  to  be  observed  in  a  drop  of  blood  mounted 
in  the  usual  manner  for  microscopical  examination  and  allowed  to 
stand  for  some  time  in  a  moist  chamber ;  they  appear  as  very  delicate 
straight  interlacing  threads  which  occupy  the  interspaces  between 
the  corpuscles  and  frequently  radiate  from  a  common  centre,  con- 
taining a  group  of  partially  broken-down  blood-platelets. 

Additional  minute  particles  are  to  be  seen  in  human  blood, 
regarding  the  nature,  source,  and  significance  of  which  much  has 
been  surmised  and  but  little  definitely  established.  These  include 
the  small  colored  disks,  the  microcytes  or  the  haematoblasts  of 
Hayem,  according  to  whose  authority  they  constitute  an  important 
source  of  the  red  corpuscles  ;  by  others  they  are  regarded  as  sep- 
arated portions  of  the  ordinary  red  cells.  Other  minute,  colorless, 
often  highly  refracting,  granules  are  encountered  floating  in  the 
liquor  sanguinis ;  such  are  the  elementary  particles  of  Zimmer- 
mann  and  the  granules  of  Max  Schultze.  These  particles  differ 
in  nature  as  well  as  in  source ;  some  probably  are  derived  from  the 
disintegration  of  the  white  corpuscles  and  of 
the    blood-platelets,    others    from   that   of   the  Fig- 

endothelial  plates  of  the  vascular  channels, 
while  many  represent  fatty  granules  absorbed 
during  digestion  or  taken  up,  possibly,  in  the 
course  of  pathological  processes. 

Blood-Crystals.    The  coloring  matter  of  the     Hamin  crystals  from  dr 
blood— the    haemoglobin— readily    crystallizes 

in  man  and  most  mammals  as  elongated,  rhombic  prisms  ;  the  haemo- 
globin crystals  of  the  squirrel  and  of  the  guinea-pig,  however,  are 
respectively  hexagonal  plates  and  rhombic  tetrahedra.  These  blood- 
crystals,  of  a  deeper  or  lighter  red  color  according  to  their  size,  often 
form  in  preparations  of  blood  which  have  been  sealed  and  allowed  to 
stand  after  the  addition  of  a  few  drops  of  water ;  the  blood  of  the 
rat  is  especially  favorable  for  their  production.  If  dried  blood  be 
treated  and  thoroughly  mixed  with  glacial  acetic  acid  (the  addition 


II2  NORMAL   HISTOLOGY. 

of  a  few  granules  of  common  salt  being  advantageous  in  the  case 
of  old  clots),  on  slightly  heating  until  bubbles  appear,  numbers  of 
dark-brown  irregular  rhombic  prisms  form.  These  are  the  haemin 
crystals  of  Teichmann,  which  are  positive  indications  of  the 
presence  of  blood,  but  have  no  value  in  the  determination  of  its 
source.  They  vary  greatly  in  size  and  considerably  in  form,  the 
peculiar  unequally-notched  ends  presented  by  the  larger  crystals 
being  quite  characteristic. 

DEVELOPMENT    OF    THE    BLOOD-CORPUSCLES. 

The  origin  of  the  colorless  blood-cells  must  be  referred  to 
the  lymphoid  tissues,  since  these  elements  are  identical  with  those 
occurring  within  the  lymph  with  which  they  are  poured  into  the 
blood-current.  The  colorless  corpuscles  appear  later  than  the  red 
cells,  the  first  ones  probably  entering  the  circulation  as  migratory 
mesodermic  elements.  The  lymphatic  or  adenoid  tissues,  however, 
undoubtedly  constitute  the  principal  sources  of  the  colorless  blood- 
corpuscles,  which  are  produced  by  the  division  of  the  numberless 
masses  of  active  protoplasm  contained  within  the  various  aggrega- 
tions of  lymphoid  tissue  throughout  the  body. 

The  multiplication  of  existing  colorless  cells  which  takes  place 
normally,  but  which  is  especially  active  under  the  stimulus  of  patho- 
logical conditions,  accounts  for  the  origin  of  a  certain  number  of 
white  corpuscles  ;  the  division  of  the  cellular  elements  of  connective 
tissue  is  regarded  by  some  as  an  additional  source  of  these  blood- 
cells.  The  efferent  lymph-streams  passing  from  the  lymphatic  tissue, 
as  well  as  the  blood  contained  in  the  splenic  vein,  are  richer  in  color- 
less cells  than  are  the  corresponding  afferent  currents,  showing  that 
the  augmentation  is  due  to  the  new  elements  contributed  by  the 
lymphoid  tissues  through  which  the  currents  pass. 

The  origin  of  the  colored  blood-cells  is  usually  considered  as 
taking  place  during  two  epochs — before  and  after  birth.  It  must  be 
remembered,  however,  that  such  division  is  conventional  and  largely 
arbitrary,  since  the  period  at  which  the  primary  embryonic  processes 
of  such  formation  cease  and  are  replaced  by  those  maintained 
throughout  life  is  uncertain  and  variable  ;  in  man  and  mammals 
born  in  a  condition  of  advanced  development  the  production  of 
blood-corpuscles  within  the  marrow  is  instituted  before  the  termina- 
tion of  intra-uterine  life. 

Before  Birth.  The  first  blood-cells  originate  outside  the  body 
of  the  embryo,  within  the  angioblastic  cells  of  the  mesodermic 
tract  of  the  vascular  area.  Certain  cells  of  this  layer  increase  in 
size  and  undergo  proliferation  of  their  nuclei,  forming  multinucleated 
areas  known  as  the  blood-islands  of  Pander.     These  subsequently 


THE   CIRCULATORY    SYSTEM. 


113 


unite  into  an  irregular  net-work,  the  nodal  points  of  which  are  dis- 
tinguished by  an  active  production  of  new  nuclei.  Some  of  these 
acquire  protoplasm  and  later  become  the  endothelium  of  the  blood- 
vessel, while  others,  more  centrally  situated,  are  converted  into  the 
primary  blood-corpuscles,  the  intervening  tissue  undergoing 
liquefaction  to  constitute  the  blood-plasma.  These  earliest  blood- 
cells,  although  destined  to  become  the  red  corpuscles,  are  at  first 
colorless  masses  of  active  protoplasm,  provided  with  nuclei  and 
exhibiting  amoeboid  movements.  After  a  time  the  protoplasm 
gradually  acquires  the  characteristic  tinge  and  assumes  a  discoidal 
form,  the  elements  then  constituting  the  nucleated  red  blood-disks 
of  the  embryo.  The  earliest  red  cells  unquestionably  increase 
by  the  division  of  the  primary  corpuscles,  the  reproduction  being- 
attended  by  the  changes  of  karyokinesis.  This  multiplication  of 
the  early  red  corpuscles  probably  ceases  in  man  long  before  the  end 
of  gestation,  the  embryonal  colored  corpuscles  meanwhile  becoming 
smaller  and  losing  their  nuclei,  so  that  at  birth  all  the  nucleated  red 
cells  have  disappeared.  The  exact  details  of  the  metamorphosis 
from  the  embryonal  to  the  adult  form  are  still  uncertain.  There  is 
no  evidence  at  present  to  establish  the  descent  of  the  red  corpuscles 
from  the  colorless  cells,  the  two  being  distinct  elements  having 
independent  origins.  The  liver  must  probably  be  reckoned  among 
the  situations  in  which  the  formation  of  blood-cells  takes  place  during 
embryonal  life  ;  in  this  same  category  is  included  the  spleen  by  some 
authorities,  probably,  however,  with  questionable  propriety. 

After  Birth.  Of  the  many  suggested  sources  for  the  post-natal 
production  of  the  red  blood-cells,  of  which  great  numbers  must  be 
formed  constantly  to  replace  those  continually  undergoing  destruction, 
the  red  marrow  of  bones  is  undoubtedly  the  most  important. 

Among  the  more  common  elements  of  the  red  marrow,  cells  usu- 
ally are  to  be  observed  which  strongly  resemble  the  embryonal  red 
blood-corpuscles,  being  distinguished  from  the  ordinary  marrow- 
cells  by  their  haemoglobin-colored  protoplasm,  smaller  size,  and 
unstable  nuclei.  These  cells,  often  called  the  erythroblasts,  are 
undoubtedly  transitional  forms  of  red  blood-corpuscles,  the  nuclei 
disappearing  and  the  protoplasm  assuming  the  usual  appearance  of 
such  elements. 

As  to  the  source  of  the  erythroblasts,  however,  whether  they  are 
transformed  colorless  marrow-cells  or  distinct  elements,  the  descend- 
ants of  the  red  corpuscles  of  the  embryo,  much  uncertainty  still 
exists.  There  are  strong  reasons  for  regarding  the  latter  supposition 
the  true  indication  of  their  nature  and  origin,  the  production  of  the 
red  corpuscles  both  before  and  after  birth  being  thus  closely  related. 

Direct  transformation  of  the  colorless  cells,  production  within  the 


j  j,  NORMAL    HISTOLOGY. 

spleen,  and  growth  from  the  blood-platelets,  or  haematoblasts  of 
Hay  em,  have  been  advanced  from  time  to  time  as  additional  sources 
of  origin  of  the  red  blood-corpuscles.  Without  entering  upon  a 
detailed  critical  consideration  of  the  evidence  supporting  these  views, 
it  may  be  stated  that,  at  present  at  least,  they  all  lack  the  conclusive 
proof  of  unimpeachable  direct  observation.  Concerning  the  rela- 
tions of  the  "  haematoblasts"  much  confusion  exists  in  consequence 
of  the  application  of  the  term  to  different  objects  by  various  writers. 
The  exceptionally  small  red  corpuscles,  or  "  microcytes,"  together 
with  those  of  unusually  large  diameter,  may  be  regarded  as  ex- 
pressing the  extremes  of  variation  in  size  to  which  all  morphological 
elements  are  subject.  The  formative  processes  within  the  red  bone- 
marrow  may  be  regarded,  in  the  light  of  our  present  knowledge,  as 
the  most  important  source,  if,  indeed,  not  the  sole  authentic  one,  of 
the  new  red  blood-corpuscles  produced  throughout  life. 


Mention  may  be  made  in  this  place  of  the  problematic  organs  the 
so-called  arterial  glands,  which  include  the  coccygeal  and  carotid 
glands. 

The  first  of  these,  the  glandula  coccygea,  or  Luschka's  gland, 
occurs  near  the  tip,  in  front  of  the  apex,  of  the  coccyx,  associated 
with  the  middle  sacral  artery,  which  contributes  the  blood-vessels 
largely  forming  its  pea-sized  mass.  The  carotid  gland  lies  at  the 
bifurcation  of  the  common  carotid  artery,  frequently  between  the 
resulting  branches,  and  appears  as  a  somewhat  flattened  ovoid 
nodule. 

These  peculiar  bodies  are  identical  in  structure,  both  consisting 
of  dense  arterial  net-works  surrounded  by  irregular  groups  of 
granular  polyhedral  cells,  whose  presence  suggested  the  once 
supposed  glandular  nature  of  the  organs.  The  entire  plexiform  mass 
is  invested  by  connective  tissue,  from  which  fibrous  septa  pene- 
trate between  the  vascular  structures.  Numerous  non-medullated 
nerve-fibres  are  also  present. 

The  true  nature  and  function  of  these  rudimentary  organs  are  en- 
tirely unknown,  and  probably  will  remain  so  until  the  embryology  of 
these  bodies  is  better  understood. 


THE    LYMPHATIC    SYSTEM. 


115 


CHAPTER    VIII. 

THE    LYMPHATIC   SYSTEM. 

The  lymphatic  system  consists  of  two  parts — the  lymph-channels 
and  their  contents  the  lymph,  and  the  lymphatic  tissue.  The 
former  may  be  represented  by  irregular  interfascicular  clefts  between 
the  bundles  of  fibrous  tissue  or  by  vessels  with  well-defined  walls, 
while  the  latter  may  exist  as  diffuse  adenoid  tissue,  the  simple  lym- 
phatic nodule,  or  the  complicated  compound  lymph-gland. 

THE    LYMPH-CHANNELS. 

The  lymphatic  spaces,  the  radicles  of  the  more  distinct  vessels, 
are  almost  universally  present,  since  they  exist  in  almost  every 
locality  where  connective  tissue  abounds,  forming  intercommuni- 
cating  systems   of  greater   or 

less  perfection  throughout  the  Fig.  135- 

various  organs.  The  relation 
between  the  connective  tissue 
and  the  lymph-radicles  is  very 
intimate,  and  it  may  be  assumed 
that  all  interfascicular  clefts 
are  directly  or  indirectly  con- 
nected with  the  ly7nphatics.  In 
loose  areolar  tissues,  as  the  sub- 
cutaneous, the  lymph-SDaCeS  Lymph-spaces  between  bundles  of  fibrous  tissue 
•11  J  c  J  1  r  •  1  seen  in  profile,  from  the  human  cornea  :£,£,  bundles 
are    lll-aenned.    CleitS,     irregular       of  fibrous  tissue  ;  j,  lymph-spaces  containing  flattened 

in   form   and   size,   which   are     connective-tissue  ceils. 
bounded   by   the   neighboring 

bundles  of  fibrous  tissue  and  lined  by  an  imperfect  layer  of  endo- 
thelioid  connective-tissue  cells.  In  the  denser  forms  of  fibrous 
tissue,  as  the  central  tendon  of  the  diaphragm,  cornea,  etc.,  the 
lymph-spaces  are  more  limited  and  form  well-defined  intercommuni- 
cating systems  of  canals,  or  "juice-channels  ;"  of  such  the  corneal 
spaces  and  the  bone-lacunas  are  familiar  examples. 

These  spaces  are  filled  incompletely  by  the  connective-tissue  cor- 
puscles, which  usually  are  applied  to  one  wall  of  the  cavity  to  form 
a  partial  lining.  The  number  of  cells  occupying  a  single  space 
varies  :  sometimes  several  lie  side  by  side  (kitten's  cornea)  united  by 
lines  of  cement-substance ;  in  such  cases,  after  silvering,  the  cells 
present  the  appearance  of  endothelial  plates.     The  large  serous 


u6 


NORMAL   HISTOLOGY. 


cavities,  as  the  peritoneal  or  pleural  sacs,  are,  in  principle,  but 
greatly-dilated    lymph-spaces,    lined    by  Fig 

modified  connective-tissue  cells,  the  en- 
dothelial plates,  which  by  mutual  press- 
ure  become    polygonal   in   outline ;    in- 

Fig.  136. 


Lymph-spaces  of  cornea,  surface  view :  a, 
the  spaces  within  the  ground-substance  (c)  con- 
nected by  the  minute  canals  (b),  or  canaliculi. 


Lymph  -  capillary  from 
silver-stained  mesentery  of 
frog:  a  number  of  lymph- 
corpuscles  occupy  the  deli- 
cate endothelial  tube  which 
constitutes  the  vessel. 


stead  of  a  few  cells  sufficing  for  the  formation  of  a  lining  membrane, 

as  in  the  case  of  the  minute 
Fig.  138.  lymph-space,  innumerable  ele- 

ments are  required  to  clothe 
the  large  serous  cavity. 

The  lymphatic  spaces  within 
the  connective  tissue  join  to 
form  definite  channels  at  the 
margins  of  the  fibrous  tissue, 
the  lymph  being  carried  by  the 
lymphatic  vessels  from  the 
organs  to  the  adjacent  masses 
of  adenoid  tissue,  the  lym- 
phatic glands.  The  lymph- 
vessels  immediately  succeed- 
ing the  spaces  may  be  regarded 
as  the  lymphatic  capil- 
laries, being  protoplasmic 
tubes  of  great  delicacy,  com- 
posed of  a  single  layer  of  en- 
dothelial plates. 

The  contours  of  the  lym- 
phatic vessels  are  not  uniform,  but  present  numerous  dilatations  and 
constrictions,  which  indicate  the  positions  of  the  imperfect  valves  : 


Lymphatics  of  silvered  diaphragm  of  rabbit:  s,s, 
lymph-spaces  lying  within  the  deeply-stained  ground- 
substance  ;  /,  /,  lymphatic  vessels  lined  with  endo- 
thelium and  possessing  valves  (v)  and  corresponding 
dilatations. 


THE    LYMPHATIC    SYSTEM. 


117 


Fig.  139. 


these  latter  consist  of  a  fold  of  endothelium,  strengthened  often  by  a 
minute  quantity  of  elastic  tissue. 

The  relation  of  the  lymph-spaces  to  the  capillary  blood-vessels  on 
the  one  hand  and  to  the  lymphatic  vessels  on  the  other  is  very  inti- 
mate ;  in  certain  localities,  as  in  the  omentum, 
indirect  communication  between  the  blood-vessels 
and  lymphatics  is  established  by  means  of  the 
spaces  of  the  groundwork  of  the  dense  connec- 
tive tissue  (Klein).  Many  nerve-trunks  are  en- 
closed by  perineurial  lymphatic  channels,  into 
which  the  lymph-spaces  of  the  surrounding  tissue 
open.  The  blood-vessels  of  the  central  nervous 
system,  especially  of  the  retina,  likewise  are 
surrounded  by  distinct  perivascular  lymph- 
sheaths,  formed  by  the  enlargement  and  con- 
fluence of  the  clefts  within  the  adventitia  of  the 
vessels.  In  some  membranous  structures,  notably 
the  amphibian  mesentery,  the  vessels  lie  encased 
within  distinct  endothelial  tubes. 

Lymphatic  vessels  of  large  size  have  walls 
of   considerable  thickness,    resembling  those   of 
the  veins.     In  such  vessels  three  coats  are  recog- 
nizable— the  inner,  or  endothelial,  the  middle,  or  muscular,  and 
the  outer,  or  connective  tissue.     The 
thoracic  duct  possesses  a  well-developed 
intima,  composed  of  a  considerable  layer 
of  subendothelial  connective  tissue  con- 
taining a  net-work  of  longitudinally  dis- 
posed elastic  fibres.     The  muscular  tissue 
of  the  media  is  supplemented  by  bundles 
of  involuntary  muscle  extending  length- 
wise within  the  outer  coat,  which  in  the 
vessel  under  consideration  is  particularly 
robust. 

The  lymph  contained  within  the  lym- 
phatic vessels,  like  the  blood,  consists  of 
two  parts  —  the  clear,  straw-colored 
plasma,  or  liquor  lymphae,  and  the 
cellular  elements,  the  lymph-cor- 
puscles. The  cells  of  the  lymph  are 
small  nucleated  masses  of  active  proto- 
plasm, when  at  rest  presenting  a  spherical 
form  and  measuring  about  .01  mm.  in 
diameter ;  in  their  usual  condition  of  activity,  however,  their  outlines 


Perivascular  lymphatic 
(b)  enclosing  a  small  ar- 
tery (a),  from  the  silvered 
mesentery  of  frog :  c, 
branching  lymphatic  cap- 
illary. 


Fig.  140. 


Transverse  section  of  human  tho- 
racic duct :  i,  m,  and  o,  respectively 
the  inner,  middle,  and  outer  tunics  ; 
.v,  endothelial  lining,  beneath  which 
lies  the  fibrous  stratum  containing 
net-work  of  longitudinal  elastic  fibres 
(y) ;  z,  longitudinally  disposed  bun- 
dles of  muscular  tissue  within  adven- 
titia; v,  capillary  blood-vessels. 


n8 


NORMAL    HISTOLOGY. 


are  continually  undergoing  the  changes  effected  by  amoeboid  move- 
ment. These  elements,  in  short,  possess  all  the  peculiarities  of  the 
colorless  blood-corpuscles  with  which,  in  fact,  they  are  identical. 

In  addition  to  the  lymph-corpuscles,  numerous  fatty  granules 
are  usually  present  within  the  plasma  ;  in  the  lymphatic  vessels  of 
the  intestinal  tract  the  absorption  of  fatty  matters  is  made  conspicuous 
by  the  presence  of  the  chyle,  an  emulsion  occupying  the  so-called 
lacteals,  or  chyle-vessels  ;  these  latter  are  not  distinct  tubes,  but 
only  those  portions  of  the  lymphatic  net-work  which  convey  the 
milky-looking  chyle  during  certain  stages  of  digestion. 

The  sources  of  the  lymph-corpuscles  are  those  already  con- 
sidered in  connection  with  the  colorless  cells  of  the  blood,  the  lym- 
phoid or  adenoid  tissues  of  the  body  being  unquestionably  the  most 
important  and  prolific  seats  for  the  production  of  these  elements. 
The  presence  of  a  few  cells  within  the  lymph-radicles,  between  their 
commencement  and  the  first  masses  of  adenoid  tissue  occurring  on 
their  course,  is  due  to  the  entrance  within  the  vessels  of  migratory 
cells  from  the  surrounding  connective  tissue  ;  only  after  the  lymph- 
stream  has  passed  through  considerable  masses  of  lymphoid  tissue 
do  the  corpuscles  appear  with  profusion. 


Fig.  141. 


THE    LYMPHATIC    TISSUES. 

Lymphatic,  lymphoid,  or  adenoid  tissue  usually  occurs  as 
circumscribed  masses  known  as  lymphatic  nodules  or  "glands;" 
in  certain  localities,  however,  as  in  parts  of  the 
mucous  membranes  of  the  larynx,  the  pharynx, 
the  stomach,  the  intestines,  etc.,  ill-defined 
masses  of  diffuse  lymphatic  tissue  occur. 
These  are  recognized  as  aggregations  of  small 
round  cells,  fading  away  among  the  surround- 
ing structures. 

Lymphatic  tissue,  wherever  found,  is  com- 
posed structurally  of  two  elements — the  deli- 
cate connective-tissue  reticulum,  on  the 
surface  of  the  fibres  of  which  plate-like,  often 
stellate,  connective-tissue  corpuscles  are  applied, 
and  the  small  round  cells  contained  within 
the  reticulum.  These  elements — the  lymphoid 
or  adenoid  cells — become  the  lymph-cor- 
puscles and  the  colorless  blood-cells  on  their 
escape  from  the  denser  reticulum  into  the 
lymph-current  and  their  subsequent  entrance 
into  the  blood. 

The  variations  in  the  compactness  with  which  the  cells  are  lodged 


Elements  of  adenoid  tissue 
from  partially  brushed  sec- 
tion of  lymphatic  gland  of 
child  :  a,  fibres  of  reticulum  ; 
b,  lymphoid  cells ;  c,  ex- 
panded connective  -  tissue 
plate. 


THE    LYMPHATIC    SYSTEM. 


II9 


within  the  net-work  constitute  the  denser  or  looser  forms  of 
adenoid  tissue  found  in  the  lymphatic  nodules ;  ordinarily  the  cells 
are  so  closely  placed  that  the  reticulum  is  greatly  masked,  satisfactory 
views  of  the  latter  being  obtained  only  in  sections  of  great  thinness 
or  after  the  cells  have  been  removed  by  brushing  or  by  violent 
agitation. 

The  reticulum  of  lymphoid  tissue  consists  of  intertwining 
and  anastomosing  bundles  of  connective  tissue  ;  along  the  fibrous 
trabecular,      especially 

Fig.  142. 


Fig.  143. 


'Smn 


Diffuse  lymphoid  tissue  occu- 
pying deeper  layers  of  mucosa 
of  human  stomach  :  the  lym- 
phoid cells  infiltrate  the  fibrous 
tissue  between  the  glands  with- 
out being  definitely  limited. 


Simple  lymph  -follicle  from 
conjunctiva  of  dog :  a,  lym- 
phoid tissue,  limited  by  the 
fibrous  capsule  (6) ;  c,  sur- 
rounding connective  tissue. 


at    the    nodal    points, 
flattened  plate-like  or 
stellate  connective-tis- 
sue   cells   are    applied 
after  the  manner  of  an 
imperfect     endothelial 
investment.  In  parts  of 
many  adenoid  struct- 
ures  the   delicate    re- 
ticulum   seems    to   be 
formed  by  the  union  of 
the  protoplasmic  pro- 
cesses of  the  branching 
connective-tissue  cells 
themselves ;     this    ar- 
rangement,   however, 
is  usually  only  seem- 
ing, the  cells  really  being  applied  to  the  surface  of  the  fibres  and  not 
constituting  an  integral  part  of  the  reticulum.     It  is  probable  that  in 
the  splenic  pulp  and  in  a  few  other  localities  the  processes  of  the 
stellate  cells  do  unite  to  form  protoplasmic  net-works. 

Diffuse  adenoid  tissue  represents  the  least  specialized  form  of 
the  lymphoid  structures ;  the  mucosae  of  the  digestive  and  of  the 
respiratory  tracts  afford  good  illustrations  of  the  presence  of  such 
tissue. 

Simple  lymphatic  nodules,  or  solitary  follicles,  stand  next  in 
differentiation ;  these  are  found  in  almost  all  mucous  membranes 
(those  of  the  bladder  and  of  the  sexual  organs  excepted),  while  they 
occur  in  great  numbers  in  the  respiratory  and  digestive  tracts,  the 
solitary  glands  of  the  latter  being  important  examples  of  these 
structures.  The  simple  nodules  consist  of  oval  masses  of  adenoid 
tissue,  limited  by  a  delicate  connective-tissue  wall  or  capsule,  com- 
posed of  fibrous  lamellae.  The  adenoid  tissue  of  such  simple  follicles 
presents  no  considerable  variations  in  its  arrangement,  that  occupy- 
ing the  more  central  portions  of  the  nodule,  however,  being  fre- 


120 


NORMAL   HISTOLOGY. 


quently  somewhat  less  closely  packed  than  the  tissue  at  the  periphery. 
The  afferent  lymph-vessels  conveying  the  lymph  to  the  simple 
follicles  break  up  at  the  periphery  of  the  nodule  into  branches,  which 
distribute  the  lymph  to  the  adenoid  tissue ;  corresponding  efferent 
vessels  carry  off  the  fluid  returned  from  the  lymphoid  tissue  and 
unite  to  form  larger  lymphatic  trunks. 

Compound  lymphatic  follicles,  the  lymphatic  glands  of  gross 
anatomy,   are  formed  by  the  aggregation  and  partial  fusion  of  a 

Fig.  144. 


Section  of  lymph-gland  from  child,  showing  general  arrangement  of  lymphoid  tissue  and  lymph- 
sinuses:  a,  capsule  from  which  trabeculje  (6,  b)  extend ;  c,  masses  of  dense  adenoid  tissue  composing 
the  cortical  follicles  ;  d,  the  same,  of  the  medullary  cords ;  e,  lymph-sinuses. 


Fig.  145. 


number  of  simple  nodules.      These  structures  enjoy  a  wide  distri- 
bution, and  are  represented  by  the  numerous  chains  of  deep  and 

superficial  lymph-glands,  of  which  the 
axillary  and  inguinal  glands  are  familiar 
instances. 

The  periphery  of  these  lymph-glands 
is  occupied  by  a  firm  capsule  composed 
of  fibrous  connective  tissue,  inter- 
mingled with  which,  in  the  largest 
glands,  bundles  of  involuntary 
muscle  are  sometimes  present.  At 
the  position  of  entrance  and  exit  of 
the  larger  blood-vessels  and  the  efferent 
lymphatic  trunks,  usually  opposite  the 
most  convex  surface  of  the  organ,  the 
capsule  dips  deeply  into  the  interior  of 
the  gland  and  forms  the  hilum.  The  space  included  within  the 
capsule  is  subdivided  into  a  peripheral  zone,  the  cortex,  and  a 
centrally  situated  part,  the  medulla,  which  at  the  hilum   reaches 


Section  of  lymphatic  gland  of  child, 
including  portion  of  cortex  at  periphery  : 
c,  capsule  ;  s,  loose  tissue  of  the  lymph- 
sinus  ;  /,  denser  lymph-tissue  of  the 
cortical  follicle. 


THE    LYMPHATIC    SYSTEM. 


121 


the  exterior.  The  details  of  arrangement  distinguishing  these  portions 
of  the  gland  depend  primarily  upon  the  distribution  of  the  trabeculae 
which  continue  the  tissue  of  the  capsule  into  all  parts  of  the  organ. 

The  trabeculae,  composed  of  stout  bundles  of  fibrous  tissue,  ex- 
tend from  the  inner  surface  of  the  capsule  towards  the  hilum  and 
divide  the  cortex  into  a  number  of  imperfect  spherical  compartments 
which  enclose  masses  of  adenoid  tissue,  the  cortical  follicles,  which 
correspond  to  simple  lymph-follicles.  The  continuations  of  the  tra- 
beculae towards  the  centre  of  the  gland  unite  at  much  more  frequent 
intervals  and  form  throughout  the  medulla  a  series  of  incomplete  par- 
titions which  separate  imperfect  compartments  occupied  by  elongated 
masses  of  adenoid  tissue,  the  med- 
ullary cords.  These  latter  and  FlG-  x47- 
the  cortical  follicles  constitute  one  ^cva*  ,  c  \(| 
continuous  mass  of  dense  lymphoid 


Section  of  lymphatic  gland  of  child,  in- 
cluding portion  of  medulla :  t,  part  of  tra- 
becula,  on  either  side  of  which  narrow 
lymph-sinuses  are  seen,  bounded  by  denser 
structure  of  medullary  cords  (/). 


Portion  of  human  lymph-gland,  showing  de- 
tails of  structure  :  a,  lymph-sinus  ;  b,  adenoid 
tissue ;  c,  trabecular ;  d,  coarser  reticulum  of 
lymph-sinus ;  e,  expanded  connective-tissue 
plate  applied  to  fibres  ;  f,  lymphoid  cells. 


tissue,  which  follows  the  contours  of  the  spaces  occupied,  but  does  not 
completely  fill  the  compartments  formed  by  the  fibrous  trabeculae. 

The  spaces  included  between  the  fibrous  trabeculae  and  the  masses 
of  dense  adenoid  tissue  are  occupied  by  a  very  loose  reticulum  and 
sparingly  distributed  lymphoid  cells  ;  these  channels  are  the  lymph- 
sinuses,  into  which  the  lymph  brought  by  the  peripherally-situ- 
ated afferent  vessels  is  poured  and  through  which  it  finds  its  sluggish 
course,  thus  securing  the  opportunity  of  taking  up  numerous  new 
cells  in  its  journey  through  the  organ.  The  lymph-sinuses  form  a 
freely  intercommunicating  system  of  canals  throughout  the  gland, 
beginning  at  the  periphery,  where  they  receive  the  afferent  lymph- 
vessels,  and  ending  in  the  hilum,  where  the  lymph  is  collected  and 
carried  off  by  the  efferent  trunks. 


j  22  NORMAL    HISTOLOGY. 

The  trabeculae  all  along  their  course  give  off  numerous  ramifi- 
cations ;  each  of  these  breaks  up  into  still  finer  bands,  until  the  final 
divisions  of  the  fibrous  tissue  terminate  in  the  delicate  reticulum  con- 
stituting the  supporting  framework  in  whose  meshes  the  lymphoid 
cells  are  held.  In  the  areas  of  denser  tissue  the  cells  are  so  closely- 
placed  that  the  supporting  reticulum  is  almost  completely  masked. 
The  surfaces  of  the  fibrous  bundles  and  partitions,  especially  those 
directed  towards  the  lymph-sinuses,  support  numerous  plate- like 
connective-tissue  cells,  in  places  these  elements  constituting 
almost  an  endothelial  covering. 

The  blood-vessels  supplying  the  lymphatic  glands  are  arranged 
as  two  groups  :  the  one  set  gains  entrance  at  the  periphery  and  is 
distributed  principally  to  the  capsule  and  larger  trabeculae  ;  the  other 
group  enters  at  the  hilum,  the  majority  of  the  arterial  branches  pass- 
ing directly  into  the  lymphoid  tissue,  while  a  few  follow  the  course  of 
the  larger  septa ;  these,  following  the  latter  course,  give  off  numerous 
twigs  to  the  surrounding  adenoid  tissue,  the  terminal  branches  con- 
tinuing to  the  capsule,  where  they  finally  are  distributed.  The  cap- 
illaries derived  from  the  breaking  up  of  the  arterial  twigs  entering  at 
the  hilum  especially  ramify  through  the  denser  adenoid  tissue,  avoid- 
ing the  loosely  reticulated  lymph-sinuses.  The  distribution  of  the 
nerves  passing  to  the  compound  lymphatic  glands  is  uncertain,  the 
supply  including  bundles  of  both  the  medullated  and  the  pale  fibres. 

In  addition  to  the  numerous  well-developed  compound  lymphatic 
follicles,  many  of  which,  as  the  mesenteric  and  the  bronchial  glands, 
reach  conspicuous  dimensions,  certain  organs  present  special  modifi- 
cations of  adenoid  tissue  ;  such  are  the  spleen  and  the  fully-developed 
thymus  body,  which  therefore  may  be  included  with  propriety  in  the 
account  of  the  lymphatic  structures. 

THE   SPLEEN. 

The  spleen  may  be  regarded  as  a  specialized  compound  lymphatic 
gland,  modified  by  the  arrangement  of  its  blood-supply.  The  organ 
is  invested  by  a  firm  capsule,  composed  of  a  dense  felt-work  of 
bundles  of  fibrous  tissue,  with  which  are  mixed  numerous  elastic 
fibres.  The  outer  surface  of  the  capsule,  with  the  exception  of  a 
limited  area,  is  covered  by  the  serous  coat  of  the  peritoneum,  the 
union  between  the  two  being  very  intimate. 

On  the  inner  surface  the  capsule  is  continuous  with  numerous 
prolongations,  the  trabeculae.  These  penetrate  deeply  into  the 
interior  from  all  sides,  and  by  the  free  union  of  their  processes  form 
a  spongy  connective-tissue  framework  throughout  the  organ, 
enclosing  an  elaborate  system  of  intercommunicating  spaces  occupied 
by  the  lymphoid  tissue. 


THE    LYMPHATIC    SYSTEM. 


123 

In  certain  animals  (dog,  cat,  hog)  the  capsule  contains  bundles  of 
involuntary  muscle  ;  these  are  only  exceptionally  present  in  man. 


Fig.  1 


Section  of  spleen  of  dog,  showing  general  structure:  a,  capsule,  from  which  trabecular  extend;  sec- 
tions of  these  latter  are  seen  in  several  places,  as  at  d  ;  b,  tissue  of  splenic  pulp  ;  c,  c,  Malpighian 
corpuscles ;  e,  sections  of  blood-vessels. 

Likewise,  bundles  of  muscular  tissue  are  constituents  of  the  trabecule 
in  many  mammals,  including  man  to  a  limited  degree ;  the  muscle- 
cells    are    distin- 


guishable from 
the  surrounding 
connective  tissue 
by  their  rod- 
shaped  nuclei. 
The  stoutest  tra- 
beculae  are  found 
at  the  hilum, 
which  corre- 
sponds to  the 
position  at  which 
the  larger  blood- 
vessels enter  and 
leave  the  organ. 

The  lymphoid 
tissue  filling  the 
intertrabecular 


Fig.  149. 


Fig.  150. 


Section  of  human  spleen,  showing 
trabecular  id)  and  fibrous  reticulum  (b) 
continued  into  the  surrounding  splenic 
pulp  ;  c,  lymphoid  cells 


Transverse  section  of  large 
trabecula  of  human  spleen  :  a, 
fibrous  tissue,  containing  a  few 
groups  of  plane  muscle-cells  (b) ; 
c,  extension  of  trabecula  into 
fibrous  reticulum ;  d,  lymph- 
corpuscles. 


spaces    exists    in 

two  forms — as  the  loose  adenoid  tissue  which,  together  with  the 


124 


NORMAL   HISTOLOGY. 


.fjV 


intimately  related  vascular  channels,  forms  the  splenic  pulp,  and  as 
the  cylindrical  or  spherical  masses  of  dense  adenoid  tissue  ensheath- 
ing  the  arteries,  constituting  the  Malpighian  corpuscles. 

The  largest  trabecular  support  the  branches  of  the  splenic  artery ; 
on  entering  at  the  hilum,  these  twigs  receive  a  strong  fibrous  invest- 
ment, or  adventitious 
Fig.  151. 


sheath,  which  accom- 
panies the  vessel  and 
becomes  gradually 
reduced  as  the  arte- 
ries diminish  in  size  ; 
finally,  this  sheath 
blends  with  the  con- 
nective-tissue frame- 
work of  the  paren- 
chyma. Many  of 
the  smaller  branches 
of  the  splenic  ar- 
tery are  deflected 
from  the  trabec- 
ulae and  enter  the 
surrounding  tissue, 
where  they  become 
ensheathed  at  irregu- 
lar intervals  by  cy- 
lindrical or  spherical  masses  of  dense  adenoid  tissue  and  constitute 
the  Malpighian  corpuscles.  The  artery  usually  pierces  the  mass 
somewhat  eccentrically,  sometimes,  however,  passing  near  the  centre. 
Numerous  small  twigs  are  distributed  to  the  tissue  composing  the 
corpuscles  ;  after  forming  a  net-work  they  eventually  open  into  the 
channels  of  the  pulp  ;  the  main  artery  of  the  Malpighian  corpuscle 
has  a  similar  destination. 

The  form  of  these  ensheathing  masses  of  adenoid  tissue  varies  in 
different  animals  ;  in  some  (guinea-pigs)  the  arteries  are  accompanied 
throughout  their  entire  course  by  a  layer  of  lymph-cells,  while  in 
others  (man,  cat)  the  investment  is  limited  to  irregularly  spherical 
masses  ;  between  these  extremes  numerous  intermediate  forms  exist. 
The  peripheral  zone  of  the  Malpighian  corpuscle  is  usually  denser 
than  the  central  part,  an  arrangement  favoring  the  sharp  demarca- 
tion of  the  body  from  the  surrounding  looser  parenchyma  ;  in  man 
the  corpuscles  are  less  clearly  defined  than  in  many  lower  animals. 

The  splenic  pulp,  which  makes  up  the  larger  part  of  the  bulk  of 
the  organ,  consists  of  a  loose  net-work  of  slender  bands  and  imperfect 
septa,  composed  of  delicate  fibres  and  broad  plate-like  connective- 


Section  of  human  spleen  cutting  transversely  a  Malpighian  cor- 
puscle :  a,  section  of  the  somewhat  eccentrically  situated  artery ; 
b,  capillaries  distributed  to  the  tissue  of  the  corpuscle  ;  /,  the  sur- 
rounding lymphoid  tissue  of  the  splenic  pulp. 


THE    LYMPHATIC    SYSTEM. 


125 


Fig.  152. 


Portion  of  channel  within  splenic  pulp  from 
human  spleen :  a,  endothelioid  connective-tis- 
sue plates  of  the  imperfect  wall  of  the  space  ; 
b,  red  blood-corpuscles  ;  c,  lymphoid  cells  ;  d, 
larger  amoeboid  elements,  containing  pigment- 
granules  ;  e,  large  multinucleated  cell. 


tissue  cells.  The  processes  of  the  latter  unite  with  one  another  to 
form  imperfect  partitions  ;  in  young  animals  multinucleated  plates 
are  frequently  encountered.  Ad- 
hering to  the  delicate  reticulum, 
partially  occluding  the  channels 
throughout  the  pulp,  are  numerous 
lymphoid  cells  or  leucocytes,  which 
are  largely  the  offspring  of  the  ele- 
ments forming  the  adenoid  tissue. 

The  spaces  of  the  splenic  pulp 
are  additionally  occupied  by  num- 
berless colored  blood-cells,  brought 
by  the  arteries  which  open  directly 
into  the  channels  within  the  pulp  ; 
the  dark-red  appearance  of  the 
organ  is  thus  explained.  As  a  re- 
sult of  the  breaking  down  of  numer- 
ous worn-out  red  blood-cells, — in 
which  process  of  destruction  the 
leucocytes  may  take  an  active  part, 
—  pigment  -  granules,  both  free 
and  within  the  lymph-cells,  are  con- 
stantly encountered.  The  splenic  pulp,  in  addition  to  giving  origin 
to  numerous  leucocytes,  in  common  with  other  lymphoid  tissues,  is 
regarded  by  some  histologists  as  the  birthplace,  as  well  as  the  ' '  grave- 
yard," of  a  certain  number  of  colored  blood-cells;  the  evidence, 
however,  upon  which  such  views  rest  is  far  from  conclusive. 

The  blood-vessels  of  the  spleen  form  an  important  part  of  the 
organ.  After  entering  at  the  hilum,  the  splenic  artery  gives  off  tra- 
becular branches  which  rapidly  diminish  in  size  by  repeated  division. 
As  already  described,  many  of  the  smaller  arteries  leave  the  septa 
and  become  ensheathed  by  the  Malpighian  corpuscles,  to  which  they 
contribute  with  capillary  net-works.  A  certain  number  of  the  arteries 
extend  the  entire  length  of  the  trabecular,  and  hence  never  become 
encased  within  the  masses  of  adenoid  tissue  ;  both  these  latter  and 
those  bearing  the  corpuscles  eventually  open  into  the  spaces  of  the 
pulp,  pouring  their  streams  of  blood  into  the  parenchyma.  The 
pulp-spaces  communicate,  on  the  other  hand,  with  a  wide-meshed 
net-work  of  venous  channels ;  the  latter  unite  to  form  a  number  of 
large  veins,  which  pass  out  at  the  hilum  in  company  with  the  principal 
arteries. 

All  the  blood  conveyed  by  the  smaller  arteries  finally  reaches  the 
spaces  of  the  splenic  pulp,  whether  directly  or  indirectly  after  having 
first  passed  through  the  tissue  composing  the  Malpighian  corpuscle ; 


126 


NORMAL    HISTOLOGY. 


the  blood  then  slowly  traverses  the  partially  obstructed  channels 
within  the  pulp  and  is  collected  by  the  venous  spaces  and  passed  on 
to  the  larger  veins,  by  which  it  escapes  from 
the  organ.  The  retarded  current  within  the 
splenic  pulp  is  favorable  to  the  removal  and 
destruction  of  the  worn-out  red  cells  and  to 
the  acquisition  of  additional  leucocytes. 
Within  the  pulp,  while  passing  from  the 
arteries  to  the  veins,  the  blood  is  probably 
not  confined  to  channels  provided  with  defi- 
nite walls,  but  comes  into  direct  relation 
with  the  lymphoid  tissue. 

The  lymphatics  of  the  spleen  are  limited 
to  the  connective-tissue  framework  of  the 
organ,  in  which  they  form  a  superficial 
plexus  in  the  deeper  layers  of  the  capsule, 
and  a  deeper  plexus  within  the  trabecular. 
The  lymphatic  clefts  within  the  adventitia 
of  the  arteries  communicate  with  the  deeper 
lymphatics  of  the  trabecular  ;  regarding  the 
definite  relations  of  the  deeper  lymphatics 
our  knowledge  is  incomplete. 

The  nerves  of  the  spleen  are  composed 
mostly  of  non-medullated  fibres,  although 
a  few  of  the  medullated  variety  are  present ; 
they  are  distributed  to  the  walls  of  the  blood- 
vessels ;  also  ganglion-cells  have  been  ob- 
served along  the  nerve-trunks. 


Diagram  of  the  relations  of 
splenic  vessels  to  the  tissue  of  the 
pulp  :  a,  v,  small  arterial  and 
venous  branches  of  splenic  vessels 
within  trabecula  {t,  t)  ;  one  twig 
of  artery  is  diverted  and  becomes 
ensheathed  by  tissue  of  the  Mal- 
pighian  corpuscle,  M :  the  remain- 
ing part  of  the  artery  follows  the 
trabecula  and  passes  directly  into 
the  spaces  of  the  pulp — in  either 
case  the  arterial  branches  termi- 
nate in  the  spaces  (/,  /)  within 
the  pulp  surrounded  by  the  lym- 
phoid tissue  (/,  /) ;  the  venous 
radicles  take  up  the  blood  and 
carry  it  from  the  spaces  of  the 
pulp  into  the  larger  venous 
trunks. 


THE   THYMUS    BODY. 

The  thymus  body  is  included  among  the  lymphatic  tissues  on  ac- 
count of  the  histological  characteristics  of  the  fully-developed  organ  ; 
in  its  early  stages,  however,  the  bulk  of  the  organ  is  epithelial  in 
nature,  being  derived  from  the  endodermic  cells  and  closely  resem- 
bling many  glands  in  its  earliest  growth.  The  rapid  invasion  of 
mesodermic  tissues,  at  a  later  period,  so  changes  the  character  of  the 
organ  that  tissues  of  a  lymphoid  type  predominate,  while  the  original 
epithelial  structures  are  reduced  to  mere  rudimentary  remains. 

The  entire  organ  usually  consists  of  two  lateral  lobes,  more  or 
less  intimately  united,  composed  of  numbers  of  lobules,  held  together 
by  the  interlobular  areolar  tissue  and  enveloped  within  the  general 
fibrous  capsule  of  the  organ.  The  irregularly  ovoid  lobules,  5-10 
mm.  in  diameter,  are  further  divided  by  connective-tissue  septa  into 
compartments,   each  of  which    includes  several  smaller  secondary 


THE    LYMPHATIC   SYSTEM.  l2y 

lobules  ;  these,  in  turn,  are  made  up  of  groups  of  the  primary  alveoli 
or  follicles.  The  latter  closely  resemble  lymph-follicles  in  structure, 
being   limited   by   a 

fibrous  envelope  giv-  Fig.  154. 

ing  off  slender  tra- 
becular, which  are 
soon  lost  in  the  deli- 
cate reticulum  of 
connective  tissue 
pervading  all  parts 
of  the  follicles.  The 
meshes  of  the  re- 
ticulum are  occupied 
by  numerous  lym- 
phoid cells,  among 
which  many  capillary 
blood  -  vessels  run. 
The  adenoid  tissue 
of  the  peripheral 
zone,  or  cortex,  of 
the  follicles  is  more 
closely  packed  with 

cells  than  that  occupying  the  centre,  or  medulla,  in  consequence  of 

which  variation  the  medulla  appears 
lighter  than  the  denser  cortex. 

Scattered  throughout  the  follicles 


Fig.  156. 


Section  of  human  thymus  body,  showing  general  arrangement  of 
follicles  :  a,  fibrous  tissue  enveloping  lymphoid  tissue  and  sending 
septa  (a')  between  the  follicles  (6)  ;  d,  interfollicular  tissue,  contain- 
ing blood-vessels  (c). 


Portion  of  the  periphery  of  one  of  the  folli- 
cles of  the  foregoing  section,  more  highly 
magnified :  a,  fibrous  tissue ;  b,  lymphoid 
tissue,  containing  numerous  capillaries  (c). 


Portion  of  the  same  follicle,  showing  corpuscles  of 
Hassall  (a),  which  represent  the  original  epithelial 
constituents  of  the  organ. 


round  or  oval  bodies  are  seen,  which  vary  greatly  in  number  and  size 
(20-175  (jl),  usually  stain  but  faintly,  and  present  an  irregularly  concen- 


I28  NORMAL    HISTOLOGY. 

trie  striation,  with  occasional  nuclei ;  these  bodies  are  the  corpuscles 
of  Hassall,  or  the  concentric  corpuscles.  They  represent  the  re- 
mains of  the  epithelial  structures  which,  as  already  stated,  in  the  early 
stages  of  the  thymus  constitute  the  principal  tissue  of  the  organ. 

The  larger  blood-vessels  of  the  thymus  run  within  the  inter- 
lobular connective  tissue,  giving  off  branches  which  penetrate  the 
follicles  and  break  up  into  a  rich  capillary  net-work  supplying  the 
adenoid  tissue  of  cortex  and  medulla. 

As  may  be  inferred  from  the  character  of  the  organ,  the  lym- 
phatics occur  in  large  numbers.  The  radicles  coming  directly  from 
the  follicles  are  received  by  the  interlobular  vessels,  which,  in  turn, 
communicate  with  the  superficial  net-work  occupying  the  surface  of 
the  organ. 

Bundles  of  nerve-fibres  accompany  the  ramifications  of  the 
arteries  and  veins,  to  the  coats  of  which  they  seem  principally  to  be 
distributed. 

The  thymus  body  reaches  its  highest  development  about  the 
second  year,  after  which  time  it  gradually  diminishes,  undergoing 
retrogressive  changes  and  absorption,  until,  by  the  eighteenth  to  the 
twenty-first  year,  the  characteristic  tissues  have  disappeared  or  have 
been  replaced  by  fibrous  connective  tissue  and  fat. 

THE   SEROUS   MEMBRANES. 

The  serous  membranes  are  intimately  related  to  the  lymphatic 
system,  since  the  cavities  which  they  enclose  form  parts  of  the  gen- 
eral lymph-tract  of  the  body ;  when  considered  in  their  widest  sig- 
nificance they  include  the  lining  of  all  cavities  clothed  with  endothe- 
lial cells  and  cut  off  from  atmosphere.  Regarded  in  a  more  limited 
and  critical  sense,  such  cavities  may  be  separated  into  certain  groups, 
following  which  the  connective-tissue  linings  may  be  divided  into  : 

a.  The  serous  membranes  pi'oper,  as  the  peritoneum,  the  pleura, 
and  the  pericardium. 

b.  The  synovial  membranes,  including  the  synovial  capsules  of 
the  joints,  the  synovial  sheaths  of  tendon,  and  the  synovial  bursae 
placed  between  opposed  movable  surfaces  to  reduce  friction. 

c.  The  endothelial  lining  of  the  vascular  system,  comprising  that 
of  the  heart,  of  the  blood-vessels,  and  of  the  lymphatics. 

d.  The  lining  of  various  spaces  developed  within  the  connective 
tissues ;  such  spaces  are  usually  small  and  provided  with  very  rudi- 
mentary linings ;  they  may  be,  however,  of  considerable  size,  as  in 
the  case  of  the  perilymphatic  spaces  of  the  internal  ear. 

The  serous  membranes  proper,  represented  by  the  peritoneum, 
the  pleura,  the  pericardium,  and  the  tunica  vaginalis,  are  all  derived 
as  constrictions  from  the  originally  single  pleuro-peritoneal  cavity 


THE    LYMPHATIC    SYSTEM. 


129 


first  formed.  In  the  closed  sacs  constituted  by  the  serous  mem- 
branes a  parietal  and  a  visceral  layer  are  always  distinguishable ; 
the  connection  of  these  with  the  subjacent  structures  is  slight  or 
intimate  according  to  the  character  and  amount  of  the  subserous 
tissue. 

Every  organ  which  projects  beyond  the  wall  of  the  serous  pouch 
into  its  cavity  must  be  enveloped  by  the  serous  membrane  to  a 
greater  or  less  degree.  When  the  organ  remains  closely  attached 
to  the  wall  of  the  body-cavity,  as  does  the  kidney,  it  obtains  only 
a  partial  serous  investment ;  where,  on  the  other  hand,  the  organ 
leaves  the  parietes  and  encroaches  upon  the  cavity,  the  serous  invest- 
ment becomes  almost  complete,  as  in  the  case  of  the  small  intestines. 
In  all  cases  the  viscera  lie  outside  the  serous  sac,  the  membrane 
which  constitutes  the  lining  of  the  space  being  pushed  before  the 
encroaching  organ  to  form  a  serous  covering  more  or  less  complete. 
The  serous  cavity  of  greatest  extent — that  of  the  peritoneum — in  the 
female  presents  an  exceptional  arrangement  in  possessing  outlets  at 
the  orifices  of  the  oviducts ;  in  this  connection,  however,  it  must 
be  remembered  that  the  oviduct  is  the  persistent  Mullerian  duct, 
which  is  only  one  of  a  number  of  tubes  formed  during  early  foetal  life 
by  evagination  of  the  primary  serous  membrane,  thus  establishing 
communication  with  surfaces  exterior  to  the 
serous  cavity.  While  such  tubes  in  the 
higher  animals  are  only  transient,  in  the 
lower  types  they  may  remain  as  permanent 
structures. 

The  serous  membranes  are  sufficiently 
thin  and  transparent  to  permit  the  color  of 
the  underlying  parts  to  be  seen  readily 
through  them ;  moderate  strength,  extensi- 
bility, and  elasticity  are  among  their  physical 
properties.  These  membranes  consist  of  the 
endothelium  covering  their  free  surface  and 
resting  upon  the  connective-tissue  stroma, 
which  constitutes  the  chief  substance  of  the 
membrane  ;  external  to  this  layer  a  variable 
amount  of  subserous  tissue  usually  is  pres- 
ent.     The  endothelium   comprises  a  single 

layer  of  the  large,  thin,  irregularly  -  polyhedral  connective  -  tissue 
plates  already  described  and  figured  in  Chapter  II. 

In  addition  to  the  minute  deeply-stained  intercellular  areas,  or 
pseudo-stomata,  true  openings,  or  stomata,  also  exist  in  the  sev- 
eral serous  membranes.  These  orifices  are  especially  well  seen  in 
silver  preparations  from  the  posterior  wall  of  the  frog's  peritoneal 

9 


Fig 


Peritoneal  endothelium  of  dog, 
silver-stained ;  several  pseudo- 
stomata  are  seen  as  dark  areas 
among  the  cells. 


j^O  NORMAL    HISTOLOGY. 

cavity  (Fig.  30);  but  they  may  be  demonstrated  also  in  the  tissues 
of  man  and  of  the  higher  animals  :  the  central  tendon  of  the  dia- 
phragm, on  which  they  were  first  discovered  in  the  peritoneum  by  von 
Recklinghausen,  offers  a  favorable  place  for  their  study.  In  addition 
to  the  stomata  occurring  in  the  peritoneum  covering  the  diaphragm, 
similar  apertures  have  been  observed  in  the  omentum,  the  pleura,  and 
the  pericardium.  The  stomata,  either  directly 
or  through  minute  canals,  lead  into  the  sub- 
jacent lymphatic  vessels  and  are  surrounded 
by  cuboidal  or  spherical  guard-cells. 

The  stroma  of  the  serous  membranes  con- 
sists of  interlacing  bundles  of  white  fibrous 
tissue,  mingled  with  elastic  fibres,  which  are 
especially  numerous  in  the  more  superficial 
Peritoneum  in  section  from     partS)  where  they  frequently  form  a  reticular 

dog:   /,    peritoneum    proper,       *  .       J  ^  J 

consisting  of  endothelium  of    layer.       1  he  interstices  between  the   fibrous 
free  surface  and  subendotheiiai     bundles  are  occupied  by  the   ground-sub- 

fibrous  stroma  containing  net-  ,        ,  _  . 

work  of  elastic  fibres;  s,  sub-  stance  ;  the  latter  alter  a  time  in  some  cases, 
peritoneal  vascular  connective  as  in  the  omentum,  suffers  local  absorption, 
interfascicular  orifices  then  partially  taking  its 
place.  The  serous  membrane,  which  in  its  earlier  condition  forms  a 
continuous  sheet,  may  become  riddled  with  apertures,  and  is  said  to 
be  fenestrated.  Where  the  ground-substance  and  stroma  are  well 
developed  and  of  considerable  thickness,  particularly  in  the  vicinity 
of  folds,  adipose  and  sometimes  lymphoid  tissue  occur  in  addition  to 
the  blood-vessels  and  lymphatics.  The  ground-substance  in  places 
where  dense  is  penetrated  by  an  intercommunicating  system  of 
lymph-spaces  opening  into  the  lymphatic  vessels  of  the  serous 
membrane.  Branched  connective-tissue  cells  are  also  frequently 
seen  with  processes  extending  between  the  endothelial  plates  of  the 
free  surface ;  such  processes  when  stained  with  silver  probably  form 
the  pseudo-stomata  already  mentioned  ;  other  protoplasmic  exten- 
sions of  the  cells  may  come  into  relation  with  the  walls  of  the  blood- 
vessels or  of  the  larger  lymphatics. 

The  subserous  layer,  where  well  developed,  is  composed  of 
loosely-arranged  bundles  of  fibro-elastic  tissue,  between  which  blood- 
vessels and  lymphatics,  with  migratory  leucocytes,  are  situated. 

The  blood-vessels  of  serous  membranes  contribute  wide-meshed 
net-works  both  to  the  layer  of  proper  stroma  and  to  the  subserous 
tissue ;  in  positions  where  tracts  of  adipose  or  of  lymphoid  tissue 
exist,  the  capillaries  form  net-works  enclosing  the  fat-sacs  or  the 
lymphoid  masses. 

The  lymphatics  of  serous  membranes  are  very  numerous,  and  are 
represented  by  the  definite  lymphatic  vessels  and  the  lymph-spaces 


THE    LYMPHATIC   SYSTEM. 


131 


within  the  ground-substance  ;  by  means  of  the  stomata  and  the  minute 
passages  leading  from  them  the  lymphatics  communicate  with  the 
serous  cavities,  while,  on  the  other  hand,  they  join  with  the  wide, 
irregular  lymph-channels  within  the  subserous  tissue. 

The  nerves  supplying  these  membranes  are  limited,  those  which 
are  present  being  largely  derived  from  the  sympathetic  system,  com- 
posed of  pale,  non-medullated  fibres  destined  chiefly  for  the  blood- 
vessels. The  few  fibres  passing  into  the  substance  of  the  membrane 
form  a  loose  reticulum  throughout  its  deeper  layers,  from  which  finer 
fibrillae  extend  beneath  the  surface. 

The  synovial  membranes,  which  constitute  a  second  group  of 
serous  membranes,  include  the  lining  of  the  clefts  developed  within 
the  connective  tissue  (mesoderm)  surrounding  opposed  movable 
surfaces,  embracing  the  capsules  enclosing  the  articulating  surfaces 
of  the  various  joints,  the  synovial  sheaths  in  which  the  tendons  glide, 
and  the  bursal  sacs  interposed  between  surfaces  ;  these  varieties  of 
synovial  membranes  are  known  respectively  as  the  articular,  the 
vaginal,  and  the  vesicular.  Synovial  membranes  differ  from 
the  serous  in  the  character  of  their  secretion  ;  that  of  the  former — 
the  synovia — is  a  glairy,  viscid  fluid,  resembling  the  white  of  egg, 
well  adapted  for  the  lubrication  of  the  opposed  parts,  and  contains 
fat   particles,   lymphoid  cells,   and    degenerated   endothelial   plates. 


Fig.  159. 


1      ^ 

Section  of  synovial  membrane  at  edge  of  articular  surface  :  s,  s,  tissue  of  synovial  membrane  bearing 
villous  projections  (v,  v) ;  x,  position  at  which  tissues  of  membrane  become  continuous  with  those  of 
periphery  of  cartilage  \f,  group  of  fat-cells  ;/,  fibrous  tissue  constituting  peripheral  zone  of  cartilage  (c) . 


The  secretion  moistening  serous  membranes  is  thinner,  watery,  and 
less  suited  to  the  reduction  of  friction. 

The  articular  synovial  membranes  surround  the  joints,  tightly 
embracing  the  bones  and  enclosing  them  within  their  sacs,  but  do 
not  extend  over  the  articulating  surfaces,  which  are  composed  of 


132 


NORMAL    HISTOLOGY. 


naked  cartilage,  over  whose  surfaces  of  contact  not  even  the  imper- 
fect endothelial  covering  is  continued ;  tendons  or  other  structures 
traversing  the  joint-cavity  receive  an  investment  of  the  synovial 
membrane.  The  marginal  zone,  embracing  the  attachment  of  the 
membrane  to  the  cartilage,  is  marked  by  the  gradual  alteration  of 
the  tissues  of  the  synovial  membrane  to  assume  the  characters  first 
of  fibro-cartilage,  and  finally  of  the  typical  articular  cartilage  of  which 
the  membrane  then  seems  a  part. 

The  synovial  sacs,  originating  as  clefts  within  the  mesoderm 
surrounding  the  extremities  of  the  young  bones,  exhibit  a  structure 
corresponding  to  slightly  condensed  connective  tissue.  The  mem- 
brane is  composed  chiefly  of  closely-felted  bundles  of  fibrous  tissue, 
mingled  with  elastic  fibres,  containing  the  usual  connective-tissue 
elements ;  the  free  surface  of  the  membrane  possesses  an  imperfect 
covering  of  connective-tissue  cells,  which,  when  closely  placed, 
as  in  the  younger  tissue,  present  the  characters  of  an  endothelium  ; 
when  less  densely  arranged,  they  retain  their  processes  and  appear 
as  branched  elements,  resembling  those  of  other  dense  fibrous  tissues  ; 
in  the  vaginal  membranes  the  cells  are  often  elongated  to  correspond 
with  the  axis  of  the  sheath. 

Cleft  folds  of  the  synovial  membrane  project  into  the  serous  cavity 
as  the  Haversian  fringes  ;  they  are  free  processes  of  the  membrane 
containing  vascular  loops  and,  in  the  larger  ones,  fat ;  the  smaller 
secondary  fringes,  or  villi,  often  present  as  finger-like  processes 
attached  to  the  edges  of  the  larger  folds,  contain  no  blood-vessels, 
but  consist  principally  of  small,  irregularly-round  cells,  separated  by 
a  scanty  intercellular  substance.  In  some  cases  these  villi  enclose  a 
denser  core,  which  consists  of  fibrous  bundles  ;  occasionally  the  entire 
villus  is  formed  of  fibro-cartilage,  the  superficial  round  cells  being 
wanting. 

Blood-vessels  are  quite  numerous  within  the  synovial  mem- 
branes, as  well  as  in  the  subjacent  tissues,  nearly  all  parts  of  the 
joints  being  generously  supplied.  Many  of  the  Haversian  fringes 
contain  vascular  tufts,  while  the  termination  of  the  blood-vessels 
around  the  margin  of  the  cartilages  is  marked  by  vascular  loops 
possessing  greatly  dilated  terminal  arches. 

The  nerves  of  the  synovial  membranes,  by  no  means  numerous, 
form  a  loose  plexus  beneath  the  free  surface  ;  in  connection  with  the 
joints,  peculiar  special  nerve-endings,  the  articular  end-bulbs  of 
Krause,  have  been  found  attached  to  the  nerve-fibres ;  Pacinian 
corpuscles  have  likewise  been  observed  in  relation  with  the  synovial 
membranes. 

The  serous  surfaces  lining  the  blood-vessels  and  the  lymphatic 


THE    LYMPHATIC    SYSTEM. 


133 


channels  and  spaces  have  been  considered  in  connection  with  their 
respective  systems. 

The  development  of  the  lymphatic  system  in  all  its  parts 
involves  the  mesoderm  alone.  Very  early  in  the  life-history  of  the 
embryo,  shortly  after  the  appearance  of  the  three  blastodermic  layers, 
the  mesoderm  undergoes  cleavage  into  two  leaves,  the  separation 
affecting  the  mesodermic  layer  on  either  side  as  far  as  the  lateral 
margin  of  the  uncleft  axial  band.  The  resulting  sheets  of  meso- 
dermic tissue  become  the  parietal  layer  (somatopleuric)  and  vis- 
ceral layer  (splanchnopleuric)  ;  the  former  clings  to  the  ectoderm 
to  become  the  future  wall  of  the  body-cavity,  while  the  latter  adheres 
to  the  entoderm  to  form  the  wall  of  the  digestive  tube. 

The  space  included  between  these  leaves  is  the  primitive  body- 
cavity,  or  ccelom,  and  the  mesodermic  tissue  forming  its  imme- 
diate wall  becomes  dif- 
ferentiated into  a  special  Fig.  160. 
lining  —  the   mesothe- 
lium — whose    elements 
are  the  ancestors  of  the 
later  endothelium. 

The  fully-formed  se- 
rous  membranes, 
represented  by  the  peri- 
toneum, the  pleura,  the 
pericardium,  and  the 
tunica  vaginalis,  are  all 
derived  as  constric- 
tions from  the  common 
pleuro  -  peritoneal  sac, 
or  body  -  cavity,  first 
formed,  the  subdivision 
of  which  into  the  above- 
mentioned  special  serous  compartments  occurs  secondarily  and  at  a 
much  later  period. 

Bearing  in  mind  the  origin  of  the  primary  lining  of  the  serous 
membranes,  the  claims  of  endothelium  to  near  kinship  with  con- 
nective tissue  must  be  admitted  ;  likewise,  the  reasons  for  regarding 
endothelium  as  distinct  in  nature  from  epithelium  will  be  appreciated. 
Inasmuch  as  the  epithelium  of  the  genito-urinary  tract  is  derived 
indirectly  from  the  mesoderm,  it  is  related  genetically  to  the  endo- 
thelium of  the  abdominal  serous  membrane. 

In  the  course  of  the  differentiation  and  growth  of  the  fibrous  con- 
nective tissue,  clefts  appear  within  the  ground-substance  between  the 
bundles  of  young  tissue,  which  become  the  lymph-spaces  of  the 


Transverse  section  of  ten-day  rabbit  embryo,  showing  the 
cleavage  of  the  mesoderm  and  the  formation  of  the  primary 
body-cavity :  E,  ectoderm ;  M,  M,  the  letters  occupy  the 
body-cavity  and  have  the  parietal  (/)  and  visceral  (v)  layers 
of  the  cleft  mesoderm  respectively  above  and  below  them  ;  the 
immediate  lining  of  the  cavity  constitutes  the  mesothelium  ; 
En,  entoderm  ;  N,  neural  canal ;  c,  notochord ;  s,  s,  cavities 
within  the  somites — really  parts  of  the  body-cavity ;  a,  one  of 
the  paired  primitive  aortse. 


!34  NORMAL    HISTOLOGY. 

maturer  stages.  The  formation  of  the  lymphatic  vessels  takes 
place  in  a  manner  very  similar  to  that  by  which  the  blood-channels 
are  produced.  The  protoplasmic  net-works  established  by  the  united 
processes  of  the  connective-tissue  corpuscles  are  at  first  solid  ;  sub- 
sequently they  acquire  a  lumen  and  become  converted  into  a  series 
of  protoplasmic  tubes,  the  nuclei  of  whose  endothelial  plates  are  de- 
rived from  the  proliferated  nucleus  of  the  original  elements.  The 
earliest  lymph-corpuscles  are,  probably,  migrated  mesoblastic 
cells  which  have  entered  the  young  vessels.  The  additional  coats 
of  the  larger  lymphatic  trunks  are  derived  from  the  condensation  and 
differentiation  of  the  surrounding  young  connective  tissue. 

The  development  of  the  lymphoid  tissue  occurs  at  a  rela- 
tively late  period.  The  position  of  the  future  lymph-gland  is  indi- 
cated by  a  cleft  or  fissure  which  appears  within  the  mesoderm  and 
completely  isolates  the  gland-area  on  all  sides  except  that  destined 
to  become  the  future  hilum,  where  the  tissue  devoted  to  the  produc- 
tion of  the  gland  and  the  surrounding  mesoderm  are  continuous. 
The  development  of  the  lymphoid  tissue  is  marked  by  increased 
numbers  and  greater  compactness  of  the  mesodermic  elements  ;  the 
supporting  reticulum,  the  capsule,  and  other  details  of  the  adenoid 
tissue  appear  later. 

The  development  of  the  spleen  begins  about  the  commence- 
ment of  the  third  month,  some  time  after  the  pancreas  has  become 
defined ;  a  condensation  of  the  mesodermic  cells,  lymphoid  in 
character,  within  the  primitive  omentum,  or  the  mesogastrium,  in 
the  near  vicinity  of  the  pancreas,  is  the  earliest  indication  of  the 
future  organ.  The  lymphoid  aggregation  first  established  is  sup- 
plemented by  the  elements  lying  beneath  the  peritoneum,  which 
differentiate  into  elongated  spindle-cells  especially  devoted  to  the 
formation  of  the  trabeculae  and  connective-tissue  framework. 
Numerous  blood-vessels  soon  grow  into  the  splenic  tissue,  the  sub- 
sequent accumulations  of  lymphoid  cells  within  the  tissue  around 
some  branches  of  the  arteries  giving  rise  to  the  Malpighian  cor- 
puscles. 

The  history  of  the  development  of  the  thymus  body  demon- 
strates an  origin  markedly  at  variance  with  the  character  of  the  fully- 
formed  organ,  since,  notwithstanding  the  pronounced  lymphatic  type 
of  the  tissue  constituting  almost  the  entire  body  when  most  complete, 
its  structure  in  the  earliest  stages  corresponds  entirely  to  embryonal 
epithelium  which  is  derived  as  the  direct  outgrowth  of  the  ento- 
derm. The  first  trace  of  the  thymus  body  appears  as  a  cylindrical 
bud  of  entodermic  tissue  springing  on  either  side  from  the  third 
pharyngeal  pouch,  or  inner  visceral  furrow.  The  epithelial  nature 
of  the  early  thymus  is  for  some  time  very  evident,  the  original  cell- 


THE    LYMPHATIC   SYSTEM.  l^ 

mass  appearing  also  similar  to  the  earliest  stage  of  a  glandular  area  ; 
repeated  division  rapidly  converts  the  at  first  simple  cylindrical  aggre- 
gation into  a  complex  figure,  in  which  an  elongated  main  part  bears 
numerous  lateral  branches.  At  a  later  period  the  surrounding  meso- 
derm becomes  richer  in  cells  and  more  compact  and  grows  into  the 
original  epithelial  structure,  the  result  of  which  invasion  is  the  final 
complete  atrophy  and  disappearance  of  the  epithelial  constituents, 
with  the  exception  of  the  inconspicuous  but  constant  corpuscles  of 
Hassall,  which  alone  bear  witness  to  the  primary  epithelial  nature 
of  the  organ. 


136 


NORMAL    HISTOLOGY. 


CHAPTER    IX. 


MUCOUS  MEMBRANES  AND  GLANDS. 


Fig.  161. 


All  passages  and  cavities  directly  or  indirectly  communicating 
with  the  exterior  of  the  body  and  the  atmosphere  are  lined  by 
mucous  membranes. 

These  structures  consist  of  two  parts :  the  connective-tissue 
stroma,  or  tunica  propria,  and  the  epithelial  covering ;  the 
outer  surface  of  the  connective-tissue  layer  is  quite  usually  special- 
ized to  form  an  extremely  delicate  basement-membrane,  or  mem- 
brana  propria,  which  thus  separates  the  epithelium  from  the  under- 
lying tissue  and  forms  a  third  constituent  of  the  mucous  membrane. 

The  basement-membrane  is  often  scarcely 
demonstrable  as  a  distinct  layer,  while 
in  certain  organs,  as  many  glands  or  the 
hair-follicles,  it  is  highly  developed. 

The  epithelium  of  mucous  surfaces 
varies  both  in  character  and  in  arrange- 
ment, as  already  described  in  Chapter  II. 
The  proper  substance  or  stroma  of  the 
mucous  membrane  consists  of  a  felt-work 
of  bands  of  fibrous  connective  tissue 
together  with  net-works  of  elastic  fibres  ; 
these  latter  may  be  so  plentiful  that  an 
especial  elastic  layer  is  formed,  as  in  parts 
of  the  respiratory  tract.  Numerous  con- 
nective-tissue cells  lie  between  or  upon 
the  fibrous  bundles,  the  flattened  plate- 
like cells  forming  in  many  places  par- 
tial linings  for  the  interfascicular  lymph- 
spaces  found  throughout  this  layer. 
Not  infrequently  the  surface  of  the 
connective-tissue  stroma  is  beset  with  numerous  elevations  or  papillae, 
over  which  the  epithelium  extends.  Such  irregularities,  when  slight, 
may  be  present  without  impressing  the  free  surface  of  the  mucous 
membrane,  since  the  epithelial  layer  completely  fills  the  depres- 
sions between  the  elevations  :  when  very  pronounced,  the  papillae 
or  folds  of  the  connective  tissue  produce  such  conspicuous  sculpt- 
urings  of  the  surface  as  the  papillae  of  the  tongue  or  the  rugae  of 
the  vagina. 


Diagram  of  a  typical  mucous  mem- 
brane :  e,  epithelium  of  free  surface  con- 
tinuing into  the  glandular  depression 
to  become  the  secreting  cells  ;  b,  base- 
ment-membrane separating  epithelium 
and  connective-tissue  stroma;  s,  s, 
fibro-elastic  tissue  of  tunica  propria; 
v,  blood  vessels  forming  net-works 
beneath  epithelium  and  around  gland. 


MUCOUS  MEMBRANES  AND  GLANDS. 


137 


Fig.  162. 


Mucous  membranes  may  be  invaded  to  a  greater  or  less  degree 
by  lymphoid  cells,  as  in  many  localities  in  the  digestive  tract ; 
sometimes,  as  in  the  villi  of  the  small  intestine,  the  tissue  assumes 
still  more  closely  the  lymphoid  type,  a  delicate  connective-tissue 
reticulum  supporting  the  lymphoid  cells. 

The  membrana  propria,  or  basement-membrane,  usually 
appears  as  a  delicate  homogeneous  line  beneath  the 
epithelium.  It  must  be  regarded  as  a  modification 
of  the  connective  tissue,  and  when  well  developed, 
after  suitable  staining  with  silver,  appears  as  a  more 
or  less  complete  covering  of  flattened,  endothelioid 
cell-plates.  The  deeper  layers  of  the  mucous  mem- 
brane fade  away  into  the  surrounding  areolar  tissue 
or  into  the  adjacent  submucosa  ;  sometimes,  how- 
ever, the  mucosa  is  limited  by  a  delicate  zone  of 
involuntary  muscle,  the  muscularis  mucosae, 
consisting  often  of  two  distinct,  although  delicate, 
layers  of  muscle-cells. 

Mucous  membranes  are  usually  provided  with 
glands,  which  in  their  simplest  type  are  depressed  portions  of  the 
general  mucous  surface,  lined  with  modified  epithelium — the  secreting 
cells.  A  single  cell  may  constitute  an  entire  gland,  instances  of  such 
arrangement  being  found  in  the  unicellular  glands  of  the  lower 
forms  ;  the  familiar  goblet-cells  are,  in  fact,  such  structures  ;  it  is, 
however,  the  more  developed  forms  of  secreting  apparatus  which 
the  term  ' '  gland' '  usually  represents. 

Glands  are  of  two  chief  varieties,  tubular  and  saccular,  each 
of  these   occurring   as  simple  and  compound.      Simple  tubular 


Plate-like  endothe- 
lioid connective-tissue 
cells  constituting  base- 
ment-membrane. 


Diagram  illustrating  the  forms  of  glands :  A ,  simple  tubular ;   B,  compound  tubular ;   C,  modified 
(coiled)  tubular;  D,  simple  saccular;  E,  compound  saccular,  or  racemose. 

glands  are  frequent,  the  peptic  glands  and  the  mucous  follicles  of 
the  intestines  being  well-known  examples.  Compound  tubular 
glands  vary  in  complexity,  from  a  simple  bifurcation  of  the  fundus, 


i38 


NORMAL    HISTOLOGY. 


Fig.  164. 


as  in  many  pyloric  or  uterine  glands,  to  the  intricate  arrangement 
of  the  tubules  of  the  kidney  or  the  testicle. 

Simple  saccular  glands  do  not  occur  in  the  higher  animals,  but 
are  conspicuous  in  the  lower  types,  as  in  the  integument  of  am- 
phibians. Compound  saccular,  or  racemose,  glands,  on  the 
other  hand,  are  represented  in  man  and  mammals  by  such  important 
organs  as  the  pancreas  and  the  salivary  glands. 

In  the  least  complex  type  of  gland,  the  simple  tubular,  the  two 
fundamental  parts  of  all  glands  are  distinguishable  in  their  primi- 
tive form  :  these  are  the  deeper  actively  secreting  portion,  the  fundus, 
and  the  superficial  division,  or  duct,  through  which  the  products  of 
the  secreting  cells  escape.  Dilatation  of  the  fundus  of  the  primitive 
type  produces  the  simple  saccular  gland  ;  division  of  the  fundus  and 
of  part  of  the  duct  originates  the  compound  tubular  variety  ;  repeated 
cleavage  and  subdivision  of  the  duct,  with  accompanying  expansion 
of  the  associated  terminal  tracts,  lead  to  the  production  of  the  com- 
pound saccular,  or  racemose,  type. 

The  tubular  glands  may  exist  as  perfectly  straight  cylindrical 
depressions ;  more  usually,  however,  the  tubes  are  somewhat  wavy 

or  tortuous  :  when  the  torsion  of  the  fundus 
reaches  its  highest  expression,  such  modi- 
fications as  the  coiled  sweat-glands  result. 

Glandular  epithelium  is  the  direct  de- 
rivative of  the  cells  covering  the  adjacent 
mucous  membrane,  so  modified  and  special- 
ized as  to  adapt  it  to  the  requirements  of 
the  several  parts  of  the  gland.  In  simple 
tubular  follicles  the  cells  of  the  adjacent 
free  surface  pass  into  those  lining  the  neck 
of  the  gland  with  little  change  ;  cells  of  the 
increased  size  and  spherical  form  become 
more  pronounced  towards  the  fundus,  where 
the  elements  assume  the  characters  of  se- 
creting epithelium.  The  cells  lining  the 
upper  part  of  the  duct  of  such  glands  not 
infrequently  exhibit  a  distinctly  imbricated 
arrangement ;  this  is  well  seen  in  the  peptic 
glands. 

The  greater  complexity  of  the  racemose 
glands  resulting  from  the  system  of  freely 
branching  excretory  tubes  renders  the  recognition  of  several  parts 
desirable.  These  are,  towards  the  ducts,  proceeding  from  the  ter- 
minal compartments,  the  alveoli  or  acini,  the  intercalated  or 
intermediate  tubules,  the  intralobular  tubes,  the  interlobular 


Tubular  glands  :  A,  simple  tubu- 
lar crypt  from  human  small  intes- 
tine ;  B,  compound  tubular  gland 
from  pyloric  end  of  human 
stomach. 


MUCOUS    MEMBRANES    AND    GLANDS. 


139 


ducts,  and  the  excretory  ducts,  which  latter  usually  unite  to  form 
a  single  common  duct  of  large  size. 

At  the  open  end  of  the  acinus  the  lining  cells  of  the  latter  become 
flattened  or  cuboidal,  and,  together  with  the  basement-membrane, 
are  directly  continuous  with  the  similar  structures  forming  the  walls  of 
the  narrow  intermediate  tubule  ;  the  latter  succeeds  the  acinus  as 
the  continuation  of  the  narrow  intercellular  clefts  of  several  adjacent 
acini,  and,  after  a  longer  or  shorter  course  as  a  delicate  narrow- 
lumened  canal,  passes  into  the  intralobular  tube.  The  distinctive 
characters  of  the  latter  are  its  larger  lumen  and  the  columnar  epithe- 
lium, many  cells  of  which  exhibit  a  distinct  vertical  striation  through- 
out the  peripheral  zone  next  the  basement-membrane.  The  branch- 
ing intralobular  tubes,  on  emerging  from  the  lobular  tissue,  join  to 
form  the  interlobular  duct  which  occupies  the  connective  tissue  lying 
between  and  holding  together  the  divisions  of  the  glandular  sub- 


Fig.  16= 


Section  of  racemose  gland  showing  relation 
of  glandular  tissue  to  origin  of  duct :  x,  acini 
lined  with  secreting  cells  which  are  directly 
continuous  with  those  of  the  intermediate 
tubule  (2) ;  y,  interlobular  connective  tissue. 


Section  of  the  human  parotid  gland 
showing  the  interlobular  tissue :  s,  s,  se- 
creting cells  of  surrounding  acini ;  d,  inter- 
lobular duct ;  v,  blood-vessels  within  the 
fibrous  tissue ;  g,  group  of  ganglion-cells. 


stance.  The  interlobular  ducts  are  clothed  with  simple  columnar 
cells,  which  form  a  passive  lining  to  the  canal  for  the  conveyance  of 
the  secretions  of  the  more  active  parts  of  the  gland.  Towards  the 
free  surface  of  the  mucous  membrane  the  interlobular  ducts  unite  to 
form  the  chief,  often  single,  excretory  duct  of  large  lumen,  whose 
walls  for  a  variable  distance  from  the  point  of  discharge  are  covered 
with  epithelium  similar  to  that  covering  the  adjoining  mucous  surface  ; 
this  is  soon  replaced,  however,  by  the  columnar  cells  which  then 
continue  into  the  smaller  tubes.  In  the  large  ducts  the  subepithelial 
tissue  is  strengthened  by  net-works  of  elastic  fibres. 

The  saccules  or  alveoli  are  limited  by  a  basement-membrane 


140 


NORMAL    HISTOLOGY. 


upon  which  rests  a  single  layer  of  irregularly  spherical  or  polygonal 
secreting  cells  ;  these  latter  do  not  entirely  fill  the  acinus,  but  leave 
an  intra-cellular  cleft,  in  which  the  system  of  tubes  for  the  conveyance 
of  the  secretions  commences. 

Glands  are  often  divided  into  serous  and  mucous,  a  differentia- 
tion depending  upon  the  peculiarities  of  the  cells  lining  the  acini  as 
well  as  upon  the  character  of  their  secretion.  The  cells  of  the  serous 
glands  are  distinguished  by  being  distinctly  granular,  generally 
spherical  in  form,  readily  and  deeply  stained  with  carmine,  and  by 
having  conspicuous  nuclei  situated  near  the  centre  of  the  cells  ;  the 
elements  of  the  mucous  glands,  on  the  contrary,   are  distended, 


Fig.  16S. 


Fig.  167. 


Serous  acini  of  human  pa- 
rotid gland  ;  the  deeply-stain- 
ing granular  cells  are  sur- 
rounded by  the  basement- 
membrane. 


Mucous  acini  of  human  lingual  gland: 
the  secreting  cells  (at,  being  loaded  with 
the  slightly-staining  secretion,  appear  clear 
and  transparent ;  c,  c,  crescentic  masses  of 
granular  cells — the  demi-lunes  of  Heiden- 
hain  ;  b,  interacinous  connective  tissue. 


very  clear  and  transparent,  slightly  stained  with  carmine,  and  have 
the  nuclei  displaced  to  the  outer  edge  of  the  cells,  not  infrequently 
immediately  beneath  the  basement-membrane.  In  the  embryonal 
pre-functionating  condition  these  two  kinds  of  glands  are  identical, 
both  as  to  mode  of  origin  and  histological  characteristics  ;  the  varia- 
tions and  the  conspicuous  differences  subsequently  appearing  depend 
on  differences  of  physiological  function  and  character  of  secretions, 
and  not  on  structural  differences  in  the  original  cells. 

Fluids  elaborated  by  the  serous  glands  are  thin  and  watery,  appear- 
ing within  the  protoplasm  of  the  secreting  cells  as  minute  dark  gran- 
ules ;  the  general  appearance  of  the  cells  depends  upon  the  number 
of  these  granules  stored  up  within  their  protoplasm.  When  a  serous 
gland  is  in  a  condition  of  rest,  the  cells  are  loaded  with  granules, 
and  consequently  they  appear  larger,  darker,  and  more  granular  ; 
after  active  secretion  the  cells  are  exhausted  and  contain  fewer 
granules,  appearing,  therefore,  smaller,  clearer,  and  less  granular. 


MUCOUS    MEMBRANES   AND    GLANDS. 


141 


The  mucous  glands  secrete  a  clear,  viscid,  homogeneous  sub- 
stance, or  mucine,  having  little  affinity  for  carmine,  but  staining 
deeply  with  haematoxylin.  During  rest  the  cells  of  such  glands 
become  loaded  and  distended  with  the  mucoid  secretion,  while  the 
nuclei  are  crowded  to  the  periphery  of  the  cells  ;  under  these  condi- 
tions the  cells  lining  the  acini  appear  clear  with  well-defined  outlines, 
and,  on  the  sides  next  the  basement-mem- 
brane, present  a  thin  zone  containing  the 
displaced  nuclei  and  granular  protoplasm. 
After  prolonged  secretion  the  exhausted 
cells  contain  relatively  little  mucoid  sub- 
stance ;  hence  the  threads  of  the  protoplasm 
are  no  longer  widely  separated,  but  are 
more  closely  placed ;  in  consequence  of 
these  changes  the  cells  assume  appearances 
resembling  those  of  the  elements  of  the 
serous  glands,  being  smaller,  darker,  and 
more  granular  than  the  cells  of  the  quies- 
cent mucous  gland. 

In  the  acini  of  mucous  glands  small  crescentic  groups  of  granular, 
deeply-staining  cells  are  often  seen  lying  between  the  clearer  elements 
and  the  basement-membrane  ;  these  are  the  crescents  of  Gia- 
nuzzi,  or  the  demi-lunes  of  Heidenhain,  the  significance  of  which 
has  caused  extended  discussion.      These  crescents  represent,  most 


Lingual  glands  from  tongue  of 
cat :  a,  b,  the  serous  and  the 
mucous  acini  containing  respec- 
tively the  granular  and  the  clear 


Fig.  170. 


B 


r  0, 

A  and  B,  serous  and  mucous  acini  in  different  stages  of  functional  activity :  r,  condition  of  rest, 
the  cells  being  gorged  with  secretion ;  a,  condition  of  exhaustion  after  great  activity  :  following  the 
discharge  of  the  secretion  the  elements  of  the  protoplasm  become  more  closely  placed,  producing  an 
appearance  of  increased  granularity. 


probably,  groups  of  quiescent  or  exhausted  cells  which  have  been 
displaced  and  crowded  to  the  periphery  of  the  acinus  by  the  dis- 
tended more  centrally  situated  active  cells.  The  view  regarding  the 
crescents  as  composed  of  young  cells  destined  to  replace  those  de- 
stroyed by  active  secretion  is  opposed  by  the  absence  of  partially 
disintegrated  cells  as  well  as  by  that  of  all  manifestations  of  cell 
division. 

The  vascular  supply  of  glands  is  always  rich.     The  larger  blood- 
vessels,   conveyed  by  the  submucosa,   send  off  branches  into  the 


Ya2  NORMAL   HISTOLOGY. 

mucosa  to  break  up  into  capillaries  which  enclose  the  tubules  and 
acini  in  close  net-works,  lying  outside  but  in  intimate  relation  with 
the  basement-membrane,  an  arrangement  favoring  the  passage  of 
substances  from  the  blood  into  the  protoplasm  of  the  secreting  cells, 
which  are  thus  placed  between  the  blood-current  on  the  one  hand 
and  the  lumen  of  the  gland  on  the  other. 

Numerous  lymphatic  spaces  are  contained  within  the  connective 
tissue  surrounding  the  acini  and  the  tubules,  some  of  the  clefts  being 
immediately  beneath  the  membrana  propria  and  in  close  relation  with 
the  gland. 

The  nerve-supply  of  glandular  structures  is  often  very  rich.  The 
nerve-trunks  accompany  the  larger  blood-vessels  in  the  submucous 
tissue  and  give  off  numerous  small  bundles  which  follow  the  smaller 
arteries  in  their  distribution  to  the  mucosa,  where  they  form  delicate 
plexuses  about  the  acini  and  the  tubules  immediately  outside  the 
basement-membrane.  The  exact  mode  of  the  final  termination  of 
the  nerves  and  their  relation  to  the  individual  secreting  cells  are  still 
matters  for  investigation  ;  whether  the  fibres  pierce  the  basement- 
membrane  to  terminate  among  the  glandular  epi- 
Fig.  171.  thelium,  while  probable,  must  be  regarded  as  still 

unproved. 

The  development  of  glands  proceeds  from 
the  epithelial  tissue  of  the  young  mucous  mem- 
brane, the  penetrating  cylinder  of  epithelium  rep- 
resenting ectodermic  or  entodermic  tissue,  except 
in  those  cases  where  the  glands  are  formed  in 
connection  with  the  parts  of  the  genito-urinary 
tract  derived  entirely  from  the  mesoderm. 

The  first  trace  of  the  glands  consists  of  a 
cylindrical  ingrowth  of  the  epithelium  into 
the  subjacent  mesodermic  tissue,  both  the  tubular 
and  the  saccular  glands  alike  starting  as  simple 
epithelial  processes.  Where,  however,  the  struct- 
ure is  destined  to  become  a  gland  of  the  racemose 
type,  the  branching  cords  of  epithelial  elements 
early  indicate  the  nature  of  the  future  gland  as 
distinguished  from  one  of  the  compound  tubular 
variety  ;  since,  in  this  case,  the  terminations  of 
the  epithelial  masses  soon  become  markedly  ex- 
panded and  club-shaped,  from  which  dilatations 
the  ultimate  divisions  or  primary  alveoli  of  the  racemose  glands  are 
extended  secondarily.  The  epithelial  cords,  at  first  solid,  later 
acquire  a  lumen  which  extends  as  far  as  the  terminal  compartments 
of  the  gland.     Sometimes,  as  conspicuously  instanced  by  the  liver, 


Developing  salivary 
gland  from  fifteen-day 
rabbit  embryo.  The 
ectodermic  ingrowth  has 
divided  into  secondary 
branches  which  termi- 
nate in  slightly  expanded 
club-shaped  ends;  e, 
epithelium  of  oral  sur- 
face ;  m,  young  connec- 
tive tissue  of  future  tunica 
propria  into  which  the 
epithelium  grows. 


MUCOUS  MEMBRANES  AND  GLANDS. 


H3 


the  primary  arrangement  of  the  gland  is  modified  by  subsequent 
changes  to  such  a  degree  that  the  original  plan  of  its  structure  is 
recognized  with  difficulty.  The  sexual  glands  are  so  highly  special- 
ized that  in  their  development  they  deviate  materially  from  the  mode 
of  the  formation  of  the  typical  secretory  organs.  Ordinarily  the 
elaborating  glandular  cells  are  ectodermic  and  entodermic  in  origin, 
while  the  basement-membranes  and  supporting  tissues  are  meso- 
dermic. 


144 


NORMAL    HISTOLOGY. 


CHAPTER    X. 


THE    DIGESTIVE   TRACT. 


Fig.  172. 


THE    MOUTH. 

The  mucous  membrane  of  the  oral  cavity  consists  of  the  epi- 
thelial covering  and  the  connective-tissue  stroma  or  tunica  propria ; 
the  deeper  layers  of  the  latter  fade  insensibly  into  the  subjacent 
tissues  which  unite  the  mucous  membrane  with  the  surrounding 
deeper  parts.  The  epithelium  lining  the  entire  oral  cavity  is  of 
the  stratified  squamous  variety,  continuous  with  the  epidermis  on 
the  one  hand  and  with  the  covering  of  the  pharynx  on  the  other. 
The  tunica  propria  is  composed  of  interlacing  bundles  of  fibrous 

connective  tissue  containing  elastic  net- 
works, and  possesses  numerous  simple 
papillae  which  encroach  on  the  epithelial 
layer,  but  do  not  appear  on  the  free 
surface  of  the  mucous  membrane.  The 
latter  is  broken  in  many  places  by  the 
openings  of  the  ducts  of  the  numerous 
glands  which  occupy  the  submucosa 
and  deeper  parts  of  the  mucosa. 

In  the  transition  of  the  skin  on  the 
lips,  where  the  skin  passes  into  the 
mucous  membrane,  the  epithelium  is 
greatly  thickened,  while  the  connective- 
tissue  layer  decreases  in  thickness  ;  the 
subepithelial  papillae  here  become  very 
prominent.  The  hair-follicles  disappear, 
but  the  sebaceous  glands  still  are  present, 
especially  near  the  angles  of  the  mouth  and  in  the  upper  lip.  The 
mucous  membrane  covering  the  cheeks  adheres  tightly  to  the  bucci- 
nator muscle,  and  possesses  small  papillae  ;  that  covering  the  gums 
is  dense,  and  contains  numerous  well-marked  papillae  beneath  the 
epithelium,  the  submucous  tissue  being  closely  united  with  the  peri- 
osteum. The  portion  covering  the  hard  palate  is  thin  and  firmly 
united  to  the  periosteum,  while  that  investing  the  soft  palate,  the 
uvula,  and  the  fauces  is  much  thicker,  less  dense,  possesses  numerous 
mucous  glands,  and,  in  many  places,  is  so  densely  crowded  with  lym- 
phoid cells  that  the  entire  mucous  membrane  assumes  the  appearance 
of  adenoid  tissue. 


Section  of  oral  mucous  membrane  of 
child  ;  the  surface  of  the  fibrous  tunica 
propria  is  broken  by  minute  papillae, 
■which  contain  the  endings  of  the  blood- 
vessels and  the  nerves.  The  papillae 
are  covered  by  the  stratified  squamous 
epithelium. 


THE    DIGESTIVE   TRACT.  ^5 

The  oral  mucous  membrane  is  thickly  beset  with  small  mucous 
racemose  glands  in  nearly  all  parts.  These  are  especially  well 
marked  on  the  lips,  the  cheeks,  the  under  surface  of  the  tongue,  and 
the  soft  palate,  constituting,  respectively,  the  labial,  the  buccal,  the 
lingual,  and  the  palatine  glands  ;  on  the  gums  and  the  hard  palate 
such  structures  are  absent  or  present  in  very  limited  numbers.  The 
acini  are  situated  within  the  deeper  layers  of  the  mucosa,  while  the 
ducts  pierce  the  superficial  layers  to  open  on  the  free  surface.  The 
squamous  epithelium  of  the  latter  is  continued  within  the  duct  usually 
as  far  as  its  first  division.  Small  lateral  isolated  groups  of  acini, 
constituting  accessory  mucous  glands,  sometimes  open  into  the  long 
narrow  excretory  duct  of  the  main  glandular  mass  during  its  journey 
to  the  free  surface. 

The  larger  blood-vessels  supplying  the  oral  mucous  membrane 
lie  within  the  submucous  tissue  and  give  off  branches  which  extend 
through  the  deeper  layers  of  the  mucosa  to  the  superficial  portions 
of  the  connective-tissue  stratum  ;  on  reaching  the  outer  boundary 
of  the  latter  the  arteries  break  up  into  rich  subepithelial  capillary 
net-works,  or,  where  papillae  are  present,  enter  the  minute  elevations 
to  supply  their  apices  with  terminal  capillary  loops.  The  capillaries 
likewise  enclose  the  acini  of  the  oral  glands. 

The  lymphatics  begin  in  the  irregular  net-work  of  interfascicular 
spaces  between  the  connective-tissue  bundles  of  the  tunica  propria  ; 
these  spaces  unite  to  form  definite  lymphatics  in  the  deepest  layers 
of  the  mucosa,  which  in  turn  are  taken  up  by  the  larger  lymph- 
vessels  of  the  submucous  tissues. 

Nerve-fibres,  largely  of  the  medullated  variety,  accompany  the 
blood-vessels,  and  form  a  subepithelial  plexus  ;  special  terminations 
— the  end-bulbs — are  found  in  the  apices  of  some  of  the  papillae, 
while  additional  numerous  tactile  corpuscles  occur  on  the  lips. 

THE   TEETH. 

In  principle,  and  among  many  of  the  lower  animals  in  fact  as  well, 
the  teeth  may  be  regarded  as  hardened  papillae  of  the  oral  mucous 
membrane. 

The  teeth  are  firmly  retained  within  their  appropriate  sockets  by 
the  close  attachment  afforded  by  the  alveolar  periosteum  which 
holds  together  the  alveolus  and  the  root  of  the  tooth.  The  perios- 
teum lining  the  alveolus  is  composed  of  dense  fibrous  tissue,  whose 
fibres  have  a  general  transverse  disposition  :  elastic  tissue  is  almost 
wanting,  nerves  and  blood-vessels  being,  however,  numerous.  At 
its  neck  the  tooth  is  especially  embraced  by  the  thickened  perios- 
teum, which  then  becomes  continuous  with  the  periosteum  covering 
the  alveolar  process  of  the  jaw  and  with  the  gum. 


146 


NORMAL   HISTOLOGY. 


The  tooth  comprises  the  dentine,  the  enamel,  and  the  cemen- 
tum. 

The  dentine,  or  ivory,  principally  contributes  the  bulk  and  the 
characteristic  form  of  the  tooth,  completely  enclosing  a  central  pulp- 
cavity,  except  where  the  narrow  nutrient  canal,  admitting  the  blood- 
vessels and  nerves  to  the  pulp,  pierces  the  apex  of  the  fang.  The 
dentine  is  composed  of  a  matrix  or  ground-substance,  which,  as 
that  of   bone,    must   be    regarded   as   modified   connective    tissue, 

formed  of  bundles  of 
fibrous  tissue  intimately 
united  and  subsequently 
impregnated  with  calca- 
reous salts. 

Piercing  the  ground- 
substance  and  appear- 
ing under  low  amplifica- 


Section   of   dried    human    tooth 

showing   portions   of  enamel   and 

Longitudinal  section  of  molar  tooth  of  kitten  :  a,  pulp-cavity,      dentine  :     a,  ground-substance    of 

continued  by  canals  {/)  to  apices  of  roots ;  d,  dentine ;  e ,  en-     dentine ;  i,  branching  dentinal  tu- 

amel ;  c,  cementum  ;  /,  alveolar  periosteum  ;  n,  neck  of  tooth  ;      bules  ;   c,  terminal  zone  of  tubules 

b,  osseous  tissue  of  jaw.  within  the  enamel  <t/). 

tion  as  a  radial  striation,  the  dentinal  tubules  extend  the  entire 
thickness  of  the  dentine  as  minute  channels  ;  they  are  seen  espe- 
cially well  in  sections  of  the  dried  tooth  in  which  the  canals  are 
filled  with  air.  Starting  from  the  pulp-surface  with  a  diameter  of 
20-26  it,  the  dentinal  tubules  pass  in  a  slightly  wavy  and  spiral 
course  through  the  dentine,  to  terminate  in  irregular  clefts,  the 
interglobular  spaces,  situated  at  the  juncture  of  the  dentine  with 
the  enamel  or  the  cementum. 


THE    DIGESTIVE    TRACT. 


H7 


The  tubules  give  off  numerous  secondary  canals  along  their  course, 
by  which  means  the  adjacent  tubules  communicate  ;  on  approaching 
the  enamel  or  the  cement  the  tubules  undergo  repeated  division, 
the  resulting  smaller  secondary  channels  corresponding  in  their 
general  direction  with  the  larger  canals. 

The  marked  parallel  curves  described  by  the  dentinal  tubules  pro- 
duce optical  effects  which  are  appreciated  as  a  coarse  striation  con- 
centric with  the  outline  of  the  pulp-cavity ;  these  appearances,  known 
as  Schrager's  lines,  may  be  seen  in  sections  with  the  unaided  eye. 
That  part  of  the  dentinal  matrix  immediately  surrounding  the  tubules 
is  especially  dense  and  resistant,  and  constitutes  the  so-called  den- 
tinal sheaths  which  may  be  isolated  by  acids.  Within  the  tubules 
lie  the  delicate  dentinal  fibres,  which  are  the  modified  processes 
of  the  connective-tissue  cells  forming  the  peripheral  layer  of  pulp- 
cells.  When  cut  across  the  tubules  appear  circular  or  slightly  oval, 
and  contain  a  minute  dot,  the  dentinal  fibre  in  transverse  section. 
Want  of  uniformity  in  the  calcification  of  the  outer  zone  of  dentine 
gives  rise  to  the  incremental  lines  of  Salter. 

The  interglobular  spaces  are  irregular  stellate  intercommuni- 
cating clefts  situated  at  the  margin  of  the  dentine,  into  which  open  on 
the  one  hand  a  number  of  dentinal  tubules  and  on  the  other  hand  the 


Fig.  175. 


Interglobular  spaces  of  dentine  from  dried 
human  tooth  :  i,  i,  spaces  into  which  certain 
dentinal  tubules  (d)  open. 


Section  of  enamel  from  dried  human 
tooth  :  a,  b,  longitudinal  and  trans- 
verse views  of  enamel  rods. 


spaces  or  the  lacunae  of  the  cementum.  Each  space  contains  a  pro- 
toplasmic body,  the  connective-tissue  cell,  the  processes  of  which 
unite  with  the  dentinal  fibres. 

The  enamel  covers  the  exposed  parts  of  the  softer  underlying 
dentine,  and  is  composed  of  irregular  4-6-sided  columns,  the  enamel 
prisms,  closely  placed  and  generally  vertical  to  the  surface  of  the 
dentine.  After  suitable  isolation  the  enamel  prisms  appear  slightly 
varicose  in  outline,  the  minute  concavities  producing  the  irregular 
dark  bands  often  seen  traversing  the  prisms.  The  prisms  are  held 
together  by  a  delicate  layer  of  cement-substance  and  grouped  into 


I48  NORMAL   HISTOLOGY. 

bundles  which  cross  one  another,  producing  the  alternate  dark  and 
light  radial  bands  seen  in  the  enamel.  The  additional  dark  lines 
extending  more  or  less  parallel  to  the  free  surface  of  the  tooth — the 
stripes  of  Retzius — are  probably  due  to  inequalities  in  growth  and 
density.  At  birth,  and  for  a  variable  time  thereafter,  the  outer  sur- 
face of  the  enamel  is  covered  by  a  delicate  but  resistant  cuticle,  the 
membrane  of  Nasmyth,  composed  of  keratose  epithelial  plates, 
the  remains  of  the  enamel  organ.  This  cuticle  is  soon  worn  away 
after  the  teeth  are  actively  used.  Next  the  dentine  numerous  clefts 
exist  for  a  short  distance  between  the  enamel  prisms  ;  they  com- 
municate with  the  interglobular  spaces  and  thus  indirectly  with  the 
dentinal  tubules. 

The  cementum,  or  crusta  petrosa,  invests  the  fang  of  the  tooth 
and  closely  resembles  in  structure  ordinary  bone  ;  the  lamellae  extend 
parallel  to  the  dentine,   as  do  likewise  the 
Fig.  177.  long  axes  of  the  bone  lacunae.     Where  the 

cementum  reaches  a  considerable  thickness, 
as  at  the  apex  of  the  root  of  the  tooth,  Ha- 
versian canals  may  exist,  although  usually 
these  are  wanting  ;  the  outer  layers  of  the 
cement  contain  fewer  and  smaller  lacunas. 
The  lacunae  communicate  with  the  dentinal 
tubules,  while  the  protoplasmic  processes 
of  their  contained  bone-cells  may  come  in 
contact  with  the  filaments  of  the  odonto- 
blasts lying  within  the  dentinal  tubules. 

The  pulp  consists  of  a  matrix  of  soft 
embryonal  connective  tissue,  in  which  nu- 
merous stellate  and  spindle  cells  form  pro- 
toplasmic net-works  by  their  anastomosing 
processes.  At  the  periphery  the  connective- 
tissue  elements  are  arranged  as  layers  of 
elongated  cylindrical  cells  perpendicular  to 
the  inner  surface  of  the  dentine,  in  contact 
with  which  they  lie  ;  these  cells  are  the 
odontoblasts,  being  the  representatives  of  the  cells  which  were 
actively  engaged  in  producing  the  dentinal  matrix.  The  protoplasm 
of  many  of  these  cells  is  prolonged  peripherally  as  delicate  threads 
into  the  dentinal  tubules,  the  processes  becoming  modified  to  form  the 
stiff  elastic  dentinal  fibres ;  centrally,  the  odontoblasts  frequently 
are  connected  with  the  stellate  connective-tissue  cells. 

The  pulp  is  richly  supplied  with  blood-vessels  and  nerves.  The 
arteries  run  in  the  long  axis  of  the  tooth,  breaking  up  into  capillary 
net-works  which  are  closest  in  the  periphery.     The  nerves  accom- 


Section  of  human  tooth  at  the 
junction  of  the  dentine  and  the 
cementum :  D,  dentine  with  its 
tubules,  which  communicate  with 
interglobular  spaces  (B)  and  with 
lacunse  of  cementum  (C). 


THE  DIGESTIVE  TRACT.  {aq 

pany  the  larger  blood-vessels  as  medullated  fibres  ;  these  give  off 
filaments  which  pass  to  the  layers  of  odontoblasts,  among  which 
they  extend  as  pale  fibres.  The  ulti- 
mate distribution  of  these  latter  is  still 
unsettled  ;  the  assertion  that  fine  fibrillar 
accompany  the  dentinal  fibres  into  the 
tubules  lacks  confirmation. 

Distinct  lymphatic  vessels  have 
not  been  demonstrated  within  the  pulp, 
although  the  clefts  within  the  matrix 
between  the  connective-tissue  fibres 
represent  the  lymph-spaces  and  are  in 
close  relation  with  the  adjacent  lym- 
phatic channels. 


DEVELOPMENT    OF   THE   TEETH. 

The  teeth  of  man  and  the  higher 
animals  are  really  exaggerated  papilla?, 
the  peripheral  parts  of  which  have 
become  specialized  and  have  under- 
gone calcification.  The  ectoderm  con- 
tributes the  enamel,  while  the  dentine, 
cementum,  and  pulp  are  derived  from 
the  mesoderm. 

A  linear  thickening  of  the  primitive 
oral  epithelium  marks  the  earliest  indication  of  the  formation  of  the 
teeth  ;  in  man  this  band  appears  before  the  end  of  the  sixth  week 
(Rose),  and  is  adherent  to  the  under  surface 
of  the  epithelial  layer.  Following  the  ex- 
pansion of  this  ectodermic  thickening  a  con- 
tinuous lateral  projection,  the  dental  ridge, 
grows  obliquely  into  the  mesodermic  tissue. 
The  dental  ridge  continues  to  grow  back- 
ward towards  the  mandibular  articulation, 
forming  an  unbroken  arch  of  ectodermic 
tissue  connected  with  the  under  side  of  the 
oral  epithelium.  The  line  of  this  attach- 
ment is  later  marked  on  the  oral  surface  by 
a  longitudinal  furrow,  the  dental  groove, 
which  has  been  long  known,  and  which  was  formerly  regarded  as  the 
initial  step  in  the  dental  development. 

While  the  dental  ridge  constitutes  a  shelf-like  common  epithelial 
invagination,  the  position  and  further  development  of  the  individual 
teeth  are  marked  by  local  thickenings  along  the  under  surface  of 


Section  of  young  tooth  of  child, show- 
ing peripheral  portion  of  pulp  and  ad- 
joining dentine:  b,  pulp-cells,  some  of 
which  send  processes  (a)  within  dentinal 
tubules  ;  c,  stroma  of  delicate  connective 
tissue ;  d,  blood-vessels. 


Fig.  179. 


Section  of  jaw  of  rabbit  embryo, 
showing  thickening  of  ectodermic 
epithelium  (ec)  from  which  dental 
ridge  (e)  begins  its  growth  into 
mesoderm  {m). 


150 


NORMAL   HISTOLOGY. 


the  ridge.      These  secondary  aggregations  are  the  first  indications 
of  the  enamel  organs  of  the  temporary  teeth.     After  the  establish- 


Fig.  180. 


Fig.  181. 


Model  of  jaw  of  human  embryo  of  40  mm.  :  r,  r,  arch  of 
increased  epithelium  constituting  dental  ridge ;/,  local  thick- 
enings corresponding  to  positions  of  future  enamel  sacs. 
(After  Rose.) 


Section  of  jaw  of  rabbit 
embryo,  showing  dental  ridge 
cut  across  :  ec,  oral  ectoderm  ; 
e,  epithelial  outgrowth  corre- 
sponding to  future  enamel 
organ  ;  m,  mesodermic  tissue. 


Fig.  182. 


ment  of  these  structures  the  ectodermic  tissue  composing  the  dental 
ridge  atrophies  and  eventually  disappears  in  the  intervals  between 
the  individual  teeth.     The  enamel  sacs  of  the  permanent  teeth  are 

formed  at  a  later  date  from  the  remains 
of  the  dental  ridge,  those  for  the  three 
permanent  molars  being  derived  from  a 
special  extension  of  the  dental  ridge 
which  grows  independently  of  ectodermic 
attachments. 

The  primitive  enamel  organ  which 
grows  from  the  dental  ridge  at  first  con- 
sists of  a  solid  cylindrical  process  of  epi- 
thelial tissue  ;  soon,  however,  the  ex- 
tremity becomes  club-shaped  and  slightly 
tortuous,  and  later  distinctly  expanded 
and  flask-shaped.  Coincident  with  these 
changes  the  surrounding  mesoderm  be- 
gins to  exhibit  proliferation  and  conden- 
sation of  its  elements,  this  differentiation 
marking  the  earliest  stage  in  the  forma- 
tion of  the  important  mesodermic  dental 
papilla,  which  very  soon  becomes  a 
conical  mass  of  closely-aggregated  meso- 
dermic elements. 

Along  with  the   growth  of  the  latter 
the  now  expanded  end  of  the  ectodermic  plug   becomes  indented 


Section  of  jaw  of  rabbit  embryo, 
showing  later  stage  of  enamel  organ, 
which  now  exhibits  differentiation 
into  outer  (6)  and  inner  (e)  cells  :  m, 
mesodermic  tissue  which  at  a  has 
undergone  already  some  condensa- 
tion ;  ec,  oral  ectoderm. 


THE  DIGESTIVE  TRACT.  jtl 

or  invaginated  to  form  an  epithelial  cap,  which  embraces  the  meso- 
dermic  dental  papilla,  and,  from  its  future  important  function,  is 
known  as  the  enamel  organ.  The  impression  of  the  dental  papilla 
upon  the  overlying  enamel  organ 
is  probably  not  to  be  attributed 
to  mechanical  obstruction  op- 
posed to  the  advancing  ecto- 
dermic  tissue,  but  has  its  cause 
in  more  deeply  lying  laws  of  ex- 
pansion along  lines  of  unequal 
growth.  As  the  invagination 
of  the  enamel  organ  progresses, 
more  and  more  of  the  dental 
papilla  becomes  covered,  until 
about  two-thirds  of  the  meso- 
dermic  cone  are  embraced  within 
the  sides  of  the  ectodermic  cap. 
The  enamel  organ  itself  under- 
goes a  differentiation  into  three 
distinct  layers  :  the  outer  layer, 
directly  continuous  for  a  long 
time  with  the  ectodermic  cells 
of  the  oral  cavity,  is  composed 
of  one  or  two  layers  of  low 
columnar  or  polyhedral  cells  ;  at 
the  point  where  they  are  reflected  to  form  the  inner,  invaginated 
part  of  the  original  epithelial  sac,  the  cells  become  elongated  and 

Fig.  i 


Section  of  jaw  of  cat  embryo  ;  the  dental  papilla 
is  seen  as  a  projecting  conical  mass  (/)  of  con- 
densed mesoderm,  whose  summit  is  enveloped  by 
the  invaginated  enamel  organ  (e) ;  ec,  oral  epi- 
thelium, still  attached  by  the  atrophic  isthmus  (a) 
with  the  enamel  organ,  whose  outer  (b),  middle 
{c),  and  inner  (d)  layers  are  differentiated  ;  e' , 
beginning  of  enamel  organ  for  permanent  tooth. 


Section  of  jaw  of  cat  embryo  with  four  developing  teeth  slightly  farther  advanced  than  in  the  pre- 
ceding stage  :  ec,  oral  epithelium ;  a,  dental  groove ;  e,  enamel  organ ;  p,  p,  dental  papillae ;  m, 
mesodermic  tissue ;  b,  b,  bone. 


distinctly  columnar,  constituting  the  inner  layer  of  the  enamel  organ 
containing  the  beautiful  enamel  cells.     The  outer  and  inner  layers 


j-2  NORMAL   HISTOLOGY. 

of  the  enamel  organ  are  separated  at  first  by  the  narrow  zone  of 
epithelial  elements  of  the  middle  layer  ;  the  cells  of  the  latter  soon 
undergo  characteristic  changes,  owing  to  an  accumulation  of  fluid, 
resulting  in  the  complete  transformation  of  the  cells,  which  become 
pressed  together  and  reduced  to  thin  plates,  the  tissue  appearing 
as  if  composed  of  irregularly  anastomosing  connective-tissue  fibres 
rather  than  of  epithelial  elements.  The  enamel  organ  retains  for  a 
considerable  time  its  connection  with  the  epithelium  of  the  oral 
cavity,  a  thin  atrophic  cord  of  cells  indicating  the  position  of  the 

former  robust  stalk.     At  the  side 
Fm. j 85.  0f  jj^g  attachment  a  lateral  cylin- 

drical projection  early  marks  the 
beginning  of  the  development  of 
the  second  enamel  organ  for 
the  permanent  tooth. 

The  columnar  cells  of  the  inner 
layer  alone  are  concerned  in  the 
production  of  the  enamel.  This 
process  consists  essentially  of  a 
gradual  deposition  on  the  inner 
\\\Vmm^^P^0y'^^^&'%nM   side  of  the  enamel  cells — that  is, 

next  the  new  dentine — 01  homo- 
geneous prisms  arranged  verti- 
cally to  the  surface  of  the  inner 
layer  of  the  enamel  organ.  The 
layer  of  enamel  increases  by  the 
addition  of  increments  deposited 
from  within  out,  the  latest-formed 
enamel  always  lying  immediately 
internal  to  the  inner  layer  of  the 
enamel  organ.  During  the  later 
stages  the  inner  and  outer  layers  are  approximated  at  the  expense 
of  the  intervening  middle  layer,  which  finally  becomes  reduced  to 
an  attenuated  stratum,  the  other  coats  of  the  enamel  sac  coming 
almost  in  actual  contact. 

During  the  changes  described  in  the  enamel  organ  the  central 
dental  papilla  is  actively  engaged  in  producing  the  dentine.  The 
top  and  sides  of  the  papilla  are  covered  by  a  layer  of  elongated, 
columnar  or  pyriform  connective-tissue  cells,  the  odontoblasts, 
which  are  the  immediate  agents  in  causing  the  deposition  of  the 
dentinal  matrix,  the  formative  process  being  similar  to  that  producing 
bone.  The  dentine  is  first  formed  at  the  apex  of  the  papilla,  and 
appears  as  a  thin  lamina  of  homogeneous  matrix  into  which  the 
delicate  processes  of  the  odontoblasts  extend,  becoming  the  dentinal 


Section  of  developing  tooth  from  cat  embryo  : 
tn,  mesodermic  tissue  condensed  in  dental  pa- 
pilla (/),  at  whose  summit  osteoblasts  (d)  are 
forming  young  dentine  (c)  ;  inner  layer  (a)  of 
enamel  organ  is  engaged  in  producing  layer  of 
young  enamel  (£) ;  e,  middle,  h,  outer  layer  of 
enamel  organ. 


THE    DIGESTIVE   TRACT. 


153 


fibres  ;  the  canals  left  within  the  matrix  to  maintain  the  nutrition 
of  the  tissue  constitute  the  dentinal  tubules,  the  homologues  of 
the  lacunae  and  canaliculi  of  bone. 
With  the  continued  growth  the  sides 
of  the  papilla  as  well  as  the  apex  be- 
come covered  by  the  layer  of  newly- 
formed  dentine  ;  the  central  part  of 
the  dental  papilla  remains,  after  all 
the  dentine  has  been  formed,  as  the 
pulp-tissue,  into  which  the  blood- 
vessels and  nerves  grow  at  a  later 
period. 

At  first  both  dentine  and  enamel 
are  soft,  the  impregnation  with  lime 
salts  occurring  subsequently ;  the 
layer  of  the  soft,  most  recently 
formed  matrix  is  readily  distin- 
guished in  stained  sections  from  the 
older  calcified  tissue.  The  cemen- 
tum,  wanting  during  fcetal  life,  is 
produced  by  the  alveolar  periosteum. 


THE   TONGUE. 

The  bulk  of  the   tongue  is  com- 


Section  of  developing  tooth  from  cat  em- 
bryo, portion  of  preceding  figure  more  highly- 
magnified  :  tn,  mesodermic  elements  consti- 
tuting pulp-tissue ;  /,  layer  of  odontoblasts  en- 
gaged in  producing  dentine  (h) ;  a  and  6,  cells 
of  middle  layer,  c  and  d,  cells  of  inner  layer 
of  enamel  organ  ;  e,  zone  of  young  enamel. 


posed  of  variously-disposed  bundles 
of  striated  fibres  of  the  lingualis, 
together  with  those  of  the  accessory  muscles,  over  the  unattached 
surfaces  of  which  the  oral  mucous  membrane  is  reflected.  The 
muscular  tissue  of  the  organ  is  arranged  in  bundles  extending  in 
three  planes  :  (i)  vertically  and  slightly  radially  (genio-hyoglossus, 
vertical  fibres  of  lingualis  and  hyoglossus)  ;  (2)  transversely  (trans- 
verse fibres  of  lingualis)  ;  (3)  longitudinally  (lingualis  superior  and 
inferior,  and  styloglossus).  A  vertical  median  partition,  the  septum 
lingualae,  divides  the  muscular  tissue  into  two  halves  ;  the  inter- 
fascicular spaces  are  filled  by  delicate  connective  tissue  and  fat,  in 
which  lie  embedded  numerous  small  lingual  glands.  Many  of  the 
muscle-fibres  find  insertion  in  the  deeper  layer  of  the  mucosa,  into 
which  their  sarcolemma  fades.  Branched  striped  muscle-fibres  are 
of  common  occurrence  in  the  tongue. 

The  mucous  membrane  forms  the  most  conspicuous  part  of  the 
organ.  That  covering  the  sides  and  inferior  surfaces  of  the  tongue 
is  thin,  containing  small  papillae  and  numerous  mucous  glands  :  on 
reaching  the  superior  surface  the  mucous  membrane  greatly  increases 
in  thickness,  and  presents  additional  conspicuous  irregularities,  the 


154 


NORMAL   HISTOLOGY. 


papillae.     The  papillae  are  of  three  kinds  :  the  filiform  or  conical, 
the  fungiform,  and  the  circum vallate. 

The  conical  papillae  are  widely  distributed,  occurring  on  all  parts 

of    the    upper    surface    of   the 
FlG'     7"  tongue.       They    consist    of    a 

conical  or  cylindrical  elevation 
of  the  connective  tissue  of  the 
mucosa,  .5-2.5  mm.  in  height, 
covered  with  a  thick  layer  of 
epithelium,  the  cells  of  which, 
as  the  most  exposed  part  of  the 
papillae,  are  partially  removed 
by  abrasion,  the  remaining  epi- 
thelium presenting  a  ragged  sur- 
face. 

The  fungiform  papillae  are 
likewise  found  on  all  parts  of 
the  tongue,  but  they  are  fewer 
in  number,  lower,  and  broader 
than  the  conical,  appearing  as 
isolated  but  distinct  red  points. 
The  connective-tissue  stalks  of 
these  papillae  are  composed  of  a 
dense  felt-work  of  fibrous  tissue, 
and  bear  secondary  papillae  on 
their  upper  surface,  the  epithelium  completely  enveloping  the  entire 
connective-tissue  core. 

The  circumvallate  papillae,  usually  eight  to  ten  in  number,  are 

placed  in  two  rows  forming  a  /\ 
at  the  posterior  part  of  the  dorsum 
of  the  tongue.  Each  consists  of 
a  large  flattened  fungiform  papilla 
surrounded  by  a  deep  furrow  and 
a  secondary  encircling  ridge  or 
wall — an  arrangement  which  has 
suggested  the  name.  The  upper 
surface  of  the  mucosa  is  beset 
with  minute  secondary  elevations, 
which,  however,  are  not  apparent 
on  the  free  surface,  being  hidden 
by  the  thick  stratum  of  covering 
epithelium. 

Lying  altogether  within  the  epithelium  lining  the  sides  of  the  deep 
circular  furrow,  the  taste -buds  appear  as  inconspicuous  oval  bodies 


Section  of  human  tongue  showing  conical  papillae: 
a,  connective  tissue  of  mucosa,  which  forms  core  of 
papillae;  i,  b,  partially  abraded  epithelium;  c, masses 
of  epithelial  cells  filling  interpapillary  recesses. 


mm/J. 

Section  of  tongue  of  child,  showing  a  fungiform 
papilla ;  the  connective-tissue  stroma  is  covered 
by  the  epithelium. 


THE   DIGESTIVE   TRACT. 


155 

occupying  almost  the  entire  thickness  of  the  epithelium.  Additional 
taste-buds  are  found  in  the  folds  in  the  vicinity  of  the  circumvallate 
papillae,  as  likewise  on  some  fungiform  papilla?.     At  the  sides  of  the 


Fig.  1 


¥j?m 


:2 


L'^*I 


Section  of  circumvallate  papilla  from  tongue  of  child :  a,  main  central  elevation,  surrounded  by  the 
annular  ridge  (b)  and  the  intervening  deep  furrow ;  c,  taste-buds  within  the  epithelium ;  d,  ducts  of 
neighboring  glands  (g,  g1 ')  ;  e,  blood-vessels. 

tongue,  just  in  front  of  the  anterior  pillars  of  the  fauces,  are  groups 
of  parallel  folds  containing  a  number  of  taste-buds  ;  these  folds  con- 
stitute the  papillae  foliatae,  which  are  highly 
developed  in  some  of  the  lower  animals,  as  in 
the  rabbit. 

The  taste-buds  are  oval,  flask-shaped 
bodies,  embedded  within  the  epithelium,  occu- 
pying usually  the  entire  thickness  of  the  latter, 
with  their  long  axes  placed  in  general  vertically 
to  the  free  surface  of  the  epithelium.  Each 
taste-bud  consists  of  an  enveloping  layer  of 
greatly-elongated  epithelial  cells,  the  cortical 
or  tegmental  cells,  which  form  a  complete 
covering,  except  over  a  small  area  correspond- 
ing to  the  superficial  pole  of  the  bud ;  at  this 
point  a  minute  canal,  the  taste -pore,  connects 
the  interior  of  the  bud  with  the  surface  of  the 
mucous  membrane. 

Within  the  epithelial  capsule  lies  a  group  of 
highly-specialized  elements,  the  gustatory 
cells.  These  neuro-epithelial  elements  appear 
as  spindle,  rod-like,  or  forked  cells,  each  being 

possessed  of  an  oval  nucleus  situated  about  the  centre  of  the  elon- 
gated body.  The  peripheral  or  outer  ends  of  these  cells  are  usually 
prolonged  with  fine  pointed  extremities,  some  of  which  terminate  in 
stiff  hair-like  processes  projecting  within  the  taste-pore  almost  as 


Taste-bud  from  circumval- 
late papilla  of  child.  The 
oval  structure  is  limited  to 
the  epithelium  (e)  lining  the 
furrow,  encroaching  slightly 
upon  the  adjacent  connec- 
tive tissue  (/) ;  o,  taste-pore 
through  which  the  taste-cells 
communicate  with  the  mucous 
surface. 


jc6  NORMAL   HISTOLOGY. 

far  as  the  free  surface.  The  inner  or  central  ends  of  the  gustatory 
cells  are  prolonged  as  slender,  sometimes  forked,  processes  ;  the 
minute  swellings  or  varicosities  which  these  extensions  often  exhibit 
are  supposed  to  indicate  the  direct  connection  of  the  neuro-epithelial 
cells  with  the  fibres  of  the  nerve  of  the  special  sense  of  taste.  It 
must  be  remembered,  however,  that  no  such  continuity  has  been,  as 
yet,  positively  demonstrated.  The  submucous  and  interfascicular 
tissue  of  the  tongue  contains  numerous  glands,  both  of  the  mucous 
and  of  the  serous  type.  The  mucous  glands  resemble  those  of 
other  parts  of  the  oral  cavity,  being  small  racemose  clusters  of  acini 
more  or  less  filled  with  clear  mucoid  secretion.  They  are  situated  in 
the  deeper  layers  of  the  submucous  tissue,  as  well  as  between  the 
bundles  of  the  muscle-fibres,  principally  in  the  posterior  part  of  the 
tongue,  although  a  group  of  small  mucous  glands  (Nuhn's)  is  found 
near  the  tip.  The  ducts  of  those  at  the  root  of  the  tongue  are  some- 
times lined  by  ciliated  epithelium. 

The  serous  glands  are  limited  to  the  immediate  neighborhood 
of  the  circumvallate  and  of  the  foliate  papillae.  The  acini  appear 
darkly  granular  and  pour  out  a  thin  watery  secretion  well  adapted  to 
aid  in  producing  gustatory  impressions. 

The  mucous  membrane  covering  the  root  of  the  tongue  contains 
also  much  adenoid  tissue,  which  occurs  either  as  diffuse  masses 
or  as  circumscribed  irregularly  spherical  lymph-follicles,  1-5  mm.  in 
diameter.  The  position  of  these  follicles  is  fre- 
quently indicated  by  slight  elevations  of  the 
mucosa,  in  the  centre  of  which  a  minute  pit  leads 
into  the  interior  of  the  lymphatic  crypt.  The 
epithelium  lining  such  recesses  is  completely  in- 
filtrated with  lymphoid  cells,  while  the  surround- 
ing diffuse  adenoid  tissue  contains  several  minute 
spherical  masses  of  denser  structure. 

Among  the  formed  elements  observed  in  the 
saliva  the  so-called  salivary  corpuscles  are 
conspicuous.  These  are  spherical  bodies,  some- 
what larger  than  the  leucocytes,  and  possess  a 
distinct  nucleus  and  minute  granules  within  the 
cell-contents  ;  under  high  amplification  these  gran- 
ules exhibit  the  agitation  characteristic  of  the  molecular  or  Brown- 
ian  motion.  The  salivary  corpuscles  are  derived  from  the  adenoid 
tissue  of  the  mouth,  and  are  really  escaped  lymphoid  cells,  which, 
in  consequence  of  the  action  of  the  saliva,  become  swollen  by  the 
imbibition  of  a  fluid  less  dense  than  the  tissue-juices  ;  they  thereupon 
exhibit  a  reaction  similar  to  that  seen  when  the  colorless  blood-cell  is 
treated  with  water. 


Fig.  191. 


Salivary  corpuscles 
from  human  saliva :  x, 
group  of  corpuscles  near 
epithelial  cells;  y,  cor- 
puscle which  has  burst, 
allowing  granules  to  es- 
cape ;  z,  salivary  cor- 
puscle highly  magnified, 
showing  granules  and 
nucleus. 


THE    DIGESTIVE    TRACT. 


157 


The  blood- supply  of  the  tongue  is  very  rich,  the  vessels  forming 
a  superficial  net-work  in  the  mucosa,  from  which  minute  twigs  as- 
cend within  the  papillae  to  terminate  at  the  summit  in  close  capillary- 
plexuses. 

The  acini  of  the  various  glands  are  surrounded  by  capillaries,  as 
are  also  the  lymph-follicles  through  the  adenoid  tissue  of  which  many 
minute  vessels  extend.  The  capillary  net-works  supplying  the  mus- 
cular tissue  follow  the  general  arrangement  and  direction  of  the  mus- 
cular fibres,  surrounding  the  latter  by  the  characteristic  rectangular- 
meshed  net-works. 

The  lymphatics  of  the  tongue  are  numerous ;  they  are  arranged 
as  a  superficial  plexus  within  the  submucous  tissue,  which  re- 
ceives    the    lymphatics    from 

the  bases  of  the  papillae ;  the  FlG-  :93- 

latter  vessels,  in  turn,  take  up 
the  smaller  trunks  having  their 

Fig.  192. 


Section  of  tonsil  of  dog  :  a,  epithelium  of 
mucous  membrane  passing  into  central  recess 
{6),  where  it  becomes  infiltrated  with  lymphoid 
cells  (e)  ;  c,  lymph-nodules  embedded  within 
diffuse  adenoid  tissue  ;  d,  neighboring  mucous 
glands. 


Section  of  tonsil  of  child  ;  the  epithelium  of  adja- 
cent surface  passes  into  the  deep  pits  which  extend 
into  the  adenoid  tissue. 


origin  in  the  numerous  interfascicular  lymph-spaces  within  the  cen- 
tral papillary  connective  tissue.  The  lymph-follicles  at  the  root  of 
the  tongue  are  well  provided  with  lymphatics,  which  surround  the 
follicles  and  give  off  radicles  to  the  adenoid  tissue. 


j  eg  NORMAL   HISTOLOGY. 

The  nerves  supplying  the  mucous  membrane — the  glossopharyn- 
geal and  the  lingual  branch  of  the  trifacial — end  either  beneath  the 
epithelium  in  the  usual  manner,  or  in  close  relation  with  the  organs 
of  special  sense — the  taste-buds.  Numerous  microscopic  ganglia  also 
occur  along  their  course,  especially  in  connection  with  the  fibres  of 
the  glosso-pharyngeal  nerve. 

THE   TONSILS. 

The  tonsils  represent  compound  lymphatic  glands,  while  con- 
siderable variation  exists  as  to  form  and  size,  each  organ  consisting 
of  an  aggregation  of  from  ten  to  eighteen  lymph-follicles,  closely 

resembling  those  found  at  the  root  of 
the  tongue  embedded  within  the  sur- 
rounding diffuse  adenoid  tissue. 
^  The  entire  mass  is  separated  from  the 
|j  adjacent  structures  on  the  attached 
borders  by  a  fibrous  capsule,  and  is 
covered  with  a  reflection  of  the  oral 
epithelium  on  the  mucous  surface,  in- 
cluding the  deep  central  pit  on  which 

The   epi- 
folds  and  de- 
pressions   oi    tnese   surfaces    is    com- 
'-§   pletely  infiltrated  with  lymphoid  cells, 
so  that  the  demarcation  between  the 


Fig 


&>  W2£k 


Numerous  mucous  glands  occupy 


Ki"^|'l^?iJ;J2S   the  immediate  vicinity  of  the  tonsils, 


Section  of  child's  tonsil,  showing  the 
details  of  the  epithelium  and  part  of  the 
lymphoid  tissue  from  preceding  figure 
under  higher  amplification. 


into  the  crypts  of  which  the  glands 
pour  their  secretion  to  mingle  with 
the  shed  epithelium  and  lymphoid 
cells  occupying  the  recesses.  Great 
numbers  of  the  escaped  lymphoid 
cells  pass  into  the  oral  cavity  to  become  salivary  corpuscles,  of 
which  the  tonsils  are  a  most  important  source. 

Blood-vessels  and  lymphatics  occur  in  large  numbers  within 
the  adenoid  tissue  ;  venous  and  lymphatic  plexuses  surround  the 
organ  receiving  the  radicles  issuing  from  the  interior.  Lymph- 
channels  encircle  the  individual  follicles,  and  afterwards  communicate 
with  the  larger  peripheral  vessels. 

Regarding  the  ultimate  distribution  of  the  nerves  little  is  defi- 
nitely known ;  fibres  have  been  traced  into  the  subepithelial 
plexus. 


THE    DIGESTIVE   TRACT. 


THE    PHARYNX. 


159 


The  pharynx  consists  essentially  of  a  fibrous  tunic,  within  which 
lies  the  mucous  membrane  with  the  submucous  tissue,  while  without 
are  arranged  the  fibres  of  the  constrictor  and  other  muscles  ;  three 
coats,  the  mucous,  the  fibrous,  and  the  muscular,  are  recognized, 
therefore,  as  forming  its  walls.  The  histological  differences  distin- 
guishing the  upper,  or  respiratory,  from  the  lower,  or  digestive, 
portion  of  the  sac  depend  largely  upon  variations  within  the  mucosa, 
especially  as  to  the  character  of  the  epithelium. 

The  upper,  respiratory  division  of  the  pharynx  is  clothed  with 
stratified  ciliated  columnar  epithelium  containing  numerous  goblet- 
cells,  while  the  part  situated  below  the  level  of  the  soft  palate  is  cov- 
ered with  stratified  squamous  cells  similar  to  those  lining  the  oral 
cavity.  The  tunica  propria,  or  stroma  of  the  mucosa,  is  formed 
of  a  felt-work  of  fibrous  bundles,  together  with  a  variable,  in  certain 
parts  large,  quantity  of  elastic  tissue.  The  subepithelial  surface  of 
the  mucosa,  where  covered  by  the  squamous  cells,  is  beset  with 
numerous  small  papillae ;  these,  however,  are  wanting  beneath  the 
ciliated  epithelium. 

Small  mucous  pharyngeal  glands  occur  in  many  places  ;  they 
are  especially  numerous  in  the  deepest  layers  of  the  mucosa  in  the 
immediate  vicinity  of  the  orifices  of  the  Eustachian  tubes,  occurring 
less  frequently  towards  the  lower  part  of  the  pharynx.  The  mucous 
membrane  contains  a  considerable  quantity  of  adenoid  tissue  ar- 
ranged as  numerous  lymph-follicles  in  the  upper  part  of  the  cavity ; 
these  follicles  closely  resemble  those  found  at  the  root  of  the  tongue, 
existing  isolated  or  in  groups.  A  conspicuous  aggregation  of  such 
structures  lies  on  the  posterior  wall  of  the  pharynx  between  the 
openings  of  the  Eustachian  tubes,  constituting  the  pharyngeal 
tonsil,  appropriately  so  named  in  view  of  the  similarity  of  its  struct- 
ure to  that  of  the  palatine  organs  of  like  name.  Some  of  the 
mucous  glands  here  also  open  into  the  central  crypt. 

The  submucous  tissue  unites  the  mucous  membrane  with  the 
fibrous  coat,  whose  dense  felt-work  of  fibro-elastic  bundles  forms  a 
structure  frequently  termed  the  pharyngeal  aponeurosis.  Its  pos- 
terior part  is  greatly  thickened  and  forms  the  raphe  to  which  the 
constrictor  muscles  are  attached. 

The  muscular  coat  is  formed  of  the  striped  fibres  constituting  the 
constrictor  and  other  muscles,  with  whose  general  arrangement  the 
disposition  of  the  muscular  tissue  agrees. 

External  to  the  muscular  coat  an  irregular  investment  of  areolar 
tissue  attaches  the  pharynx  to  the  surrounding  structures. 

The  larger  blood-vessels,  lymphatics,  and  nerve-trunks  take 


j6o  normal  histology. 

their  course  within  the  submucous  tissue,  and  send  off  branches  to 
supply  the  mucosa  in  a  similar  manner  as  in  the  mouth.  The  lym- 
phatics are  exceptionally  numerous  in  the  vicinity  of  the  lymph- 
follicles,  around  which  they  form  net-works  continuous  with  those 
of  the  nasal  cavity,  the  oesophagus,  and  the  larynx. 

The  nerves  supplying  the  pharynx,  derived  from  the  cranial  and 
sympathetic  trunks  taking  part  in  the  formation  of  the  pharyngeal 
plexus,  contain  both  medullated  and  non-medullated  fibres,  associ- 
ated with  minute  ganglia.  Small  twigs  are  given  off  from  the  larger 
branches  to  terminate  in  the  subepithelial  tissue  and  among  the  acini 
of  the  mucous  glands  and  the  lymphatic  follicles. 

The  walls  of  the  digestive  tract,  from  the  oesophagus  to  the  anus, 
are  composed  of  four  tunics — the  mucous,  the  submucous,  the 
muscular,  and  the  fibrous  or  serous.  The  muscular  coat, 
usually  thickest  and  most  rigid,  is  the  most  essential  structure  in 
maintaining  the  form  of  the  tube.  The  mucosa  is  distinguished 
by  the  highly-specialized  secreting  apparatus  which  it  contains,  as 
well  as  by  the  variations  and  the  modifications  of  its  surface  ; 
the  difference  between  the  several  divisions  of  the  digestive  tract  is 
dependent  largely  upon  the  changes  in  the  character  of  this  tunic. 

The  submucosa  loosely  connects  the  mucous  coat  with  the  mus- 
cular, and  affords  space  for  the  larger  blood-vessels,  the  lymphatics, 
and  the  nerves,  as  well  as  for  some  few  glandular  structures  and 
lymphoid  masses. 

The  fibrous  coat  gives  additional  strength  to  the  walls  of  the 
digestive  tube,  and  presents  a  smooth  external  serous  surface  in  those 
parts  of  the  tract  which  receive  a  reflection  from  the  peritoneum. 

THE    OESOPHAGUS. 

The  walls  of  the  oesophagus  comprise  four  coats — the  mucous, 
the  submucous,  the  muscular,  and  the  fibrous. 

The  mucous  membrane  is  a  continuation  of  that  of  the  pharynx, 
and  corresponds  closely  with  the  latter  in  structure. 

The  stratified  squamous  epithelium  rests  upon  the  connective- 
tissue  matrix,  the  tunica  propria,  the  inner  surface  of  which  bears 
numerous  small  papillae  completely  hidden  by  the  thick  overlying 
epithelium.  The  deeper  layers  of  the  mucosa  are  separated  from 
the  submucous  coat  by  longitudinal  bundles  of  involuntary  muscle, 
the  muscularis  mucosae ;  these  muscular  bundles,  absent  in  the 
upper  part  of  the  oesophagus,  first  appear  as  irregular  and  inter- 
rupted groups,  which  become  more  numerous  until,  from  the  middle 
of  the  tube  on,  they  form  a  continuous  longitudinally-disposed  layer. 

The  submucous  coat  is  composed  of  loosely-united  connective 
tissue,  serving  for  the  conveyance  and  support  of  the  larger  blood- 


THE   DIGESTIVE  TRACT. 


161 


vessels,  lymphatics,  and  nerves.  Within  the  submucosa  are  placed 
likewise  the  acini  of  the  mucous  glands ;  these  are  rather  more 
numerous  on  the  anterior  surface,  their  ducts  piercing  the  mucosa 
and  opening  on  the  free  surface  of  the  mucous  membrane,  being 
lined  throughout  the  greater  part  of  their  length  by  columnar  epi- 
thelium.    In  the  lower  portion  of  the  oesophagus,  particularly  about 

Fig.  195. 


Section  of  human  oesophagus :  a,  squamous  epithelium  of  surface  resting  upon  fibrous  tissue  of 
mucosa,  the  deeper  part  of  which  is  occupied  by  muscularis  mucosae  (6) ;  c,  submucous  coat,  con- 
taining glands  (A)  ;  d,  e,  respectively  circular  and  longitudinal  muscular  tunics ;  e' ,  e' ,  bundles  of 
striped  muscle-fibres. 

the  cardiac  orifice,   the  mucous  glands  are  very  plentiful  and  lie 
within  the  mucosa. 

The  muscular  tunic  consists  of  two  layers,  an  inner  circular 
and  an  outer  longitudinal,  whose  component  bundles  are  held 
together  by  the  connective-tissue  septa  which  pass  between  the  fas- 
ciculi in  all  directions.  The  character  of  the  muscular  tissue  varies 
in  the  several  portions  of  the  tube.  That  contained  within  the  wall 
of  the  upper  third  of  the  oesophagus  is  entirely  of  the  striated 
variety,  while  the  muscular  tissue  of  the  lower  third  is  exclusively 


j52  normal  histology. 

non-striped  or  involuntary  in  character  ;  in  the  middle  third 
both  kinds  exist,  the  striated  fibres  gradually  disappearing  as  the 
non-striped  fibres  increase.  The  latter  extend  highest  in  the  circular 
coat  and  somewhat  farther  in  the  anterior  than  on  the  posterior  wall. 
The  last  traces  of  voluntary  muscle  appear  as  short,  isolated  striped 
fibres  among  the  surrounding  fasciculi  of  non-striated  tissue. 

The  fibrous  coat  envelops  the  muscular  tunic  externally,  strength- 
ening the  tube  and  affording  attachment  to  the  surrounding  areolar 
tissue  connecting  the  oesophagus  with  neighboring  organs.  Con- 
siderable elastic  tissue  is  found  in  this  coat,  the  elastic  fibres  forming 
net-works  intimately  connected  with  the  bundles  of  involuntary 
muscle. 

The  larger  blood-vessels  penetrate  the  outer  coats  and  ramify 
within  the  submucous  tissue,  from  which  branches  pass  to  supply  the 
muscular  and  mucous  tunics,  the  capillaries  within  the  latter  ending 
as  net-works  within  the  inner  part  of  the  tunica  propria.  The 
lymphatics  of  the  deeper  layers  of  the  mucosa  terminate  in  the 
larger  vessels  of  the  submucosa.  Numerous  nerve-fibrillae  pass 
from  the  submucous  tunic  into  the  mucosa  to  end  beneath  the 
epithelium. 

THE   STOMACH. 

The  stomach  must  be  regarded  as  a  dilated  and  specialized  portion 
of  the  general  digestive  tube,  its  walls  consisting  of  the  four  coats 
common  to  the  other  parts  of  the  tract — namely,  the  mucous,  the 
submucous,  the  muscular,  and  the  serous  or  fibrous  tunic. 

The  mucous  membrane  is  covered  by  a  simple  columnar 
epithelium,  the  squamous  cells  of  the  oesophagus  abruptly  ter- 
minating at  the  cardiac  orifice  to  be  replaced  by  the  columnar  ele- 
ments of  the  gastric  epithelium,  many  of  which  are  goblet-cells. 
The  free  inner  surface  of  the  stomach  presents,  in  addition  to  the 
conspicuous  folds  or  rugae,  minute  inequalities  and  pits,  which 
mark  the  openings  of  the  gastric  glands  ;  the  mouths  of  the  latter 
show  as  minute  depressions,  between  which  the  intervening  por- 
tions of  the  mucosa  extend  as  apparent  elevations. 

The  gastric  glands  are  of  two  kinds — the  peptic  glands,  situ- 
ated in  the  middle  and  cardiac  thirds,  and  the  pyloric  glands, 
found  in  the  pyloric  third  of  the  stomach.  Both  varieties  are  limited 
to  the  mucosa,  extending  in  length  the  entire  thickness  of  this  coat. 

The  peptic  glands  are  slightly  wavy,  simple  tubular  depressions, 
in  which  a  duct,  a  neck,  and  a  fundus  are  recognized.  In  excep- 
tional cases  the  fundus  is  divided,  while  in  nearly  all  it  is  tortuous  or 
spiral,  its  extremity  being  often  sharply  bent  at  right  angles  to  the 
general  axis  of  the  tube.     The  columnar  epithelial  cells  of  the  ad- 


THE    DIGESTIVE    TRACT. 


163 


jacent  gastric  mucous  membrane  pass  into  the  ducts  of  the  glands 
with  little  change,  becoming  imbricated,   and,    towards  the  neck, 
shorter  and  more  spherical  in  outline.     At  the 
neck,  the  narrowest  part  of  the  tube,  the  cells  Fig.  197. 

are  more  cuboidal,  and  assume  a  columnar  or 
pyramidal  form  as  they  approach  the  fundus. 
The  chief  or  central  cells  bound  the  lumen 


Fig.  i 


Section  of  human  stomach,  showing  general  arrangement  of  its  coats  : 
a,  mucosa  containing  the  tubular  peptic  glands ;  e,  muscularis 
mucosae  separating  the  layer  of  glands  from  the  underlying  submucous 
coat  (b) ;  h,  blood-vessels ;  c,  c' ,  respectively  the  circular  and  longi- 
tudinal muscular  layers ;  d,  the  fibrous  tunic  covered  with  the  peri- 
toneum. 


Peptic  gland  from 
stomach  of  dog :  a,  wide 
mouth  and  duct  which  re- 
ceive the  terminal  divisions 
of  the  gland ;  b,  c,  neck 
and  fundus  of  the  tubes ;  e, 
central  or  chief,  d,  parietal 
or  acid,  cells. 


of  the  gland  and  form  the  bulk  of  the  glandular  epithelium.  Each 
cell  contains  a  spherical  nucleus  embedded  within  the  granular  pro- 
toplasm, whose  exact  condition  depends  upon  the  state  of  functional 
activity.  In  addition  to  the  chief  or  central  cells,  a  second  variety, 
the  parietal  or  acid  cells,  exists  in  the  peptic  glands.  As  indicated 
by  their  name,  the  parietal  cells  are  situated  in  the  periphery  of  the 
gland  immediately  beneath  the  basement-membrane,  usually  separated 
from  the  lumen  by  the  intervening  central  cells.     Minute  lateral 


X54  normal  histology. 

intercellular  clefts  or  canals  in  many  places  afford  direct  commu- 
nication between  the  parietal  cells  and  the  lumen  of  the  tube.  The 
parietal   cells   are   irregularly  distributed   from    the   fundus   to   the 

Fig.  iq8. 


Transverse  sections  of  peptic  glands  from  stomach  of  dog:  A,  plane  of  section  passes  through 
ducts  near  free  surface  ;  a,  lumen  of  glands  ;  b,  surrounding  fibrous  stroma  of  mucosa  ;  B,  plane  of 
section  passes  through  fundi  near  terminations  of  tubules  ;  the  sections  of  the  latter  are  arranged  in 
groups  separated  by  connective  tissue. 

neck  of  the  gland  ;  but  they  are  especially  numerous  in  the  vicinity 
of  the  neck.  These  cells  are  larger  than  those  lining  the  lumen, 
polygonal  or  triangular  in  outline,  and  possessed  of  a  pale,  faintly 
granular  protoplasm  surrounding  a  round  or  oval 
nucleus.  In  preparations  of  human  stomach,  the 
parietal  cells  are  not  infrequently  the  most  con- 
spicuous and  best  defined,  since  the  central  cells 
are  prone  to  disintegrate. 

On  approaching  the  pyloric  ring,  the  simple 
tubular  peptic  glands  are  gradually  replaced  by 
the  compound  glands,  until,  near  the  intestinal 
opening,  these  alone  are  present. 

The  pyloric  glands  are  characterized  by  their 
relatively  long,  wide  ducts  into  which  the  several 
divisions  of  the  body  open ;  the  tubular  com- 
partments are  wavy  and  tortuous,  and  frequently 
end  in  slightly  expanded  extremities.  The  duct 
is  lined  by  tall  columnar  epithelium,  the  cells  be- 
coming lower  and  broader  as  they  approach  the 
neck  and  towards  the  fundus.  The  cells  contain 
finely  granular  protoplasm,  and  do  not  secrete 
mucus,  but  a  thin  albuminous  fluid.  Parietal  or 
acid  cells  do  not  occur  in  the  pyloric  glands,  being 
confined  to  the  true  peptic  glands. 

The  gastric  glands,  while  very  uniformly  dis- 
tributed  through   all   parts   of  the   stomach,   are 
arranged   in    groups,    the   individual   tubules   of 
which  are  separated  by  very  delicate  partitions  of  the  connective 


C 

Portion     of 
gland   of    dog 
magnified :    a, 
central    c 
next    the 


peptic 

iog,   highly 

a,    a,   the 

chief   cells 

lumen    (c) ; 


b,  b,  the  parietal  or 
acid  cells  connected 
with  the  lumen  of  the 
tube  by  short  lateral 
branches  which  extend 
to  the  cells. 


THE   DIGESTIVE    TRACT.  x6- 

tissue,  thicker  layers  of  fibrous  tissue  enveloping  the  entire  group. 
Numbers  of  lymph- cells  are  intermingled  with  the  fibrous  tissue 
of  the  mucosa ;  in  the  vicinity  of  the  pylorus  considerable  patches 
of  diffuse  adenoid  tissue  lie  around  and  among  the   ends  of  the 

gastric  follicles  and  constitute  the 
lenticular  glands. 

The  muscularis  mucosae  oc- 
cupies the  deepest  layer  of  the 

Fig.  201. 


Fig.  200. 


Section  of  pyloric  glands  from  human  stomach  : 
a,  mouth  of  gland  leading  into  long,  wide  duct 
(6),  into  which  open  the  terminal  divisions ;  c, 
connective  tissue  of  the  mucosa. 


Section  of  pyloric  region  of  human  stomach, 
showing  irregular  mass  of  adenoid  tissue  lying 
between  the  gastric  tubules  (g'fg)  constituting  a 
lenticular  gland  ;  s,  submucous  tissue. 


tunica  propria,  and  is  composed  of  an  inner  circular  and  an  outer 
longitudinal  layer  of  non-striped  muscle ;  the  tissue  of  the  muscu- 


FlG.  202. 


Longitudinal  section  of  child's  stomach  passing  through  pyloric  orifice  :  S,  I ,  the  gastric  and  the  in- 
testinal surface  ;  /,  pyloric  glands,  which  gradually  extend  into  the  submucosa  to  become  Brunner's 
glands  (i) ;  a,  simple  follicles  of  the  intestinal  mucosa  ;  s,  submucosa  ;  t,  the  greatly  thickened  layer 
of  circular  muscle  constituting  the  pyloric  ring;  /,  longitudinal  muscular  tunic. 

laris  mucosae  extends  within  the  interglandular  septa,  often  as  far  as 
the  free  surface  of  the  mucous  membrane,  beneath  which  the  muscle- 
cells  disappear. 

The  submucosa  is  a  coat  of  considerable  thickness,  composed 
of  a  felt-work  of  fibro-elastic  bundles  of  varying  size,  but  so  loosely 
interwoven  that  the  mucosa   may  be   shifted   readily  within   con- 


1 66 


NORMAL    HISTOLOGY. 


Fig 


siderable  latitude  upon  the  underlying  muscular  tunic.  The  large 
prominent  folds,  or  rugae,  of  the  stomach  involve  both  the  mucous 
and  the  submucous  coat,  the  latter  forming  the  connective-tissue 
frame-work  of  the  elevation  over  which  the  mucosa  with  its  glands 
is  reflected.  Within  the  mesh-work  of  connective-tissue  bundles 
are  supported  the  larger  blood-vessels,  lymphatics,  and  nerves. 

The  muscular  tunic  comprises  two  principal  sheets  of  involun- 
tary muscle,  disposed  as  an  inner  circular  and  an  outer  longitudinal 
layer ;  towards  the  cardiac  end  of  the  stomach  irregular  bundles  of 
oblique  fibres  constitute  an  imperfect  third  layer.  The  pyloric 
orifice  is  guarded  by  a  fold  of  mucous  membrane  supported  by 
the  submucosa  and  strengthened  by  a  conspicuous  local  annular 
thickening  of  the  inner  circular  layer  of  muscle ;  the  outer  longi- 
tudinal muscular  layer  and  the  serous  coat  pass  over  into  the  intes- 
tinal wall  without  partici- 
pating in  the  formation 
of  this  gastro-duodenal 
valve. 

The  serous  coat  is 
composed  of  bundles  of 
fibrous  connective  tissue, 
together  with  rich  net- 
works of  elastic  fibres, 
while  the  peritoneal  sur- 
face is  covered  with  a 
single  layer  of  the  charac- 
teristic endothelial  plates. 
The  narrow  areas  included 
between  the  folds  of  the 
peritoneum  along  their 
lines  of  reflection  are,  of 
course,  devoid  of  the 
serous  covering ;  at  these 
points  the  vessels  and  the 
nerves  pass  to  and  from 
the  stomach. 


Section  of  injected  stomach  of  cat :  a,  rugse  consisting 
of  the  mucosa  and  a  core  of  submucous  tissue  (6) ;  c,  d,  the 
circular  and  longitudinal  layers  of  muscle  ;  all  the  dark 
lines  represent  the  blood-vessels  filled  with  the  carmine- 
gelatin  mass ;  the  larger  trunks  break  up  in  the  submucosa, 
sending  twigs  into  the  mucous  and  muscular  tunics. 


The  larger  arteries,  after  penetrating  the  outer  coats,  divide 
within  the  submucosa  into  smaller  branches,  one  set  of  which  pierces 
the  muscularis  mucosae  to  be  distributed  to  the  mucous  membrane, 
while  the  other  enters  the  muscular  and  serous  tunics.  The  vessels 
supplying  the  mucosa  form  a  rich  subepithelial  capillary  net- 
work, as  well  as  mesh-works  surrounding  the  gastric  glands,  the  cap- 
illaries lying  immediately  beneath  the  basement  in  close  proximity 
to  the  glandular  epithelium.     The  branches  distributed  to  the  outer 


THE    DIGESTIVE   TRACT. 


167 


layers  form  long-meshed  capillary  net-works,  from  which  the  muscle- 
bundles  and  fibrous  tissue  derive  their  supply. 

The  larger  lymphatic  trunks  accompany  the  blood-vessels  and 
form  a  coarse  plexus  within  the  submucous  tissue ;  a  much  closer 
net-work  of  smaller  lymphatics  occupies  the  deeper  part  of  the 
mucosa,  from  which  radicles  ascend  between  the  glands  to  end 
beneath  the  epithelium  in  slightly  dilated  blind  extremities. 
Peripherally -situated  lymph-vessels  drain  the  masses  of  adenoid 
tissue.  In  addition  to  the  lymphatics  of  the  mucosa,  the  larger 
vessels  of  the  submucosa  take  up  those  from  the  muscular  coat. 

The  nerves  of  the  stomach,  after  piercing  the  serous  coat,  take 
up  a  position  between  the  circular  and  longitudinal  muscular  layers, 
in  which  situation  they  form  a  rich  plexus,  consisting  of  both  medul- 


Fig.  204. 


5*-^"r  A  r-:-i"  '§tlrS< i^fi^"'^^  ' 


rv  -M 


/>-' 


%-£\  ■'■■*■■  K  ■■''•''■- 


Surface  views  of  nervous  plexuses  of  stomach  of  young  child.  A,  Auerbach's  plexus:  g,  groups 
of  ganglion-cells  ;  r,  underlying  muscular  tissue.  B,  Meissner's  plexus  :  g,  groups  of  ganglion-cells  ; 
b,  blood-vessel.    (After  Stokr.) 

lated  and  pale  fibres ;  at  the  nodal  points  of  this  net-work  numerous 
microscopic  ganglia  are  situated,  the  whole  forming  the  intramuscular 
ganglionic  plexus  of  Auerbach. 

From  this  plexus  fibres  are  distributed  to  the  serous  coat  and  to 
the  longitudinal  layer  of  muscle,  as  well  as  to  the  outer  part  of  the 
circular  layer.  The  intramuscular  net-work  is  continued  by  numerous 
small  bundles  of  fibres,  which,  after  piercing  the  inner  layer  of  cir- 
cular muscle,  and  giving  off  lateral  twigs  to  the  inner  part  of  the 
same,  enter  the  submucosa  to  form  there  a  second  ganglionic  plexus 
similar  to  the  one  lying  between  the  muscular  layers  :  this  is  the 
plexus  of  Meissner.  The  submucous  plexus  sends  off  numerous 
fibres  into  the  mucosa,  which  are  distributed  beneath  the  epithelium 


1 68 


NORMAL   HISTOLOGY. 


and  to  the  gastric  glands ;  the  exact  mode  of  termination  of  these 
nerve-fibrillae  within  the  mucosa,  however,  is  still  undetermined. 


THE    INTESTINES. 

The  four  coats  of  the  stomach  are  continued,  with  little  modifica- 
tion, into  the  mucous,  the  submucous,  the  muscular,  and  the 
serous  tunics  of  the  intestinal  wall ;  the  variations  characterizing 
the  several  divisions  of  this  tube  are  dependent  largely  upon  modi- 
fications and  specializations  of  the  mncoiis  membrane. 

The  free  inner  surface  of  the  small  intestine  is  studded  over  with 
small  cylindrical  elevations — the  villi — projecting  into  the  intestinal 

lumen  and  bathed  in  the 
FlG-  2°5-  juices  of  the  canal.      In 

addition  to  the  villi,  which 
are  found  through  the 
whole  extent  of  the  small 
intestine,  the  mucous 
membrane  is  thrown  into 
transverse  or  oblique  per- 
manent folds  —  the  val- 
vulae  conniventes  — 
which  extend  partially 
around  the  tube,  and  are 
most  marked  in  the  duo- 
denum and  the  jejunum  ; 
these  folds  increase  the 
area  of  the  mucous  sur- 
face, and  are  beset  with 
villi  the  same  as  the  sur- 
rounding parts  of  the 
mucosa.  These  projections,  the  villi  and  the  valvuke  conniventes, 
are  peculiar  to  the  small  intestine  and  serve  to  distinguish  it  from 
the  large. 

The  mucosa  is  covered  by  a  single  layer  of  columnar  epithelium 
resting  upon  the  basement-membrane.  The  prismatic  cells  contain 
finely  granular  protoplasm  and  oval  nuclei,  the  latter  being  usually 
situated  within  the  inner  half  of  the  cell.  The  outer  free  ends  of 
the  cells  are  invested  by  a  peculiar  cuticular  zone,  or  basilar  border, 
a  well-defined  continuous  band  exhibiting,  in  suitably  preserved 
specimens,  a  fine  vertical  striation.  The  significance  of  these  mark- 
ings is  still  uncertain,  especially  in  view  of  the  fact  that,  after  the 
action  of  such  reagents  as  water,  the  border  breaks  up  into  rods 
resembling  very  coarse  cilia ;  the  striation  is  regarded  by  others  as 
the  expression  of  fine  parallel  canals. 


Longitudinal  section  of  human  small  intestine,  showing 
general  relation  of  the  folds  constituting  the  valvular  conni- 
ventes to  the  mucosa  and  submucous  coat ;  the  latter  con- 
tributes the  fibrous  core  over  which  the  mucosa  with  its  villi 
and  glands  extends. 


2o6. 

WW 


1 


Bsd    rsat£==j 


a, 


I 


THE    DIGESTIVE   TRACT.  jgg 

Goblet- cells  are  numerous,  many  epithelial  elements  having  be- 
come distended  with  mucoid  secretion  :  in  carmine  preparations  the 
cells  appear  as  clear,  oval  breaks  in  the 
contour  of  the  epithelium.  While  occur- 
ring throughout  the  entire  digestive  tube, 
the  goblet-cells  are  especially  numerous  in 
the  large  intestine,  where  not  infrequently 
the  majority  of  the  epithelial  elements  are 
in  this  condition.  During  certain  stages  of 
digestion  the  protoplasm  of  the  epithelium 
may  contain  oil-drops  taken  up  from  the 
intestinal  contents.  Migratory  leuco- 
cytes are  also  found  in  the  intercellular 
clefts.  The  epithelium  rests  upon  a  mem- 
brana  propria — the  endothelium  of  Debove 
— composed  of  flattened  connective-tissue 
plates. 

The  villi  consist  entirely  of  the  tissues 
of  the  mucosa,  the  epithelium  extending 
over  the  projecting  portions  of  the  tunica 
propria  to  form  a  complete  investment  of 
the  finger-like  processes.  The  centre  of 
each  villus  is  occupied  by  the  absorbent 
chyle-vessel,  or  lacteal,  a  slightly  club- 
shaped  lymphatic  radicle,  which  ends 
blindly  near  the  apex  of  the  villus  and 
whose  walls  are  composed  of  a  single  layer 

of  endothelium.  The  tissue  surrounding  the  lacteal  and  formin 
the  bulk  of  the  projection  approaches  in 
character  quite  closely  adenoid  tissue,  con- 
sisting of  a  fibrous  reticulum  holding  many 
lymphoid  cells  within  its  meshes.  The 
central  lacteal  lies  enclosed  within  a  capil- 
lary net-work,  extending  through  the 
greater  part  of  the  villus  and  connecting  the 
afferent  arteriole  and  efferent  veins.  Imme- 
diately surrounding  the  lacteal,  and  in  inti- 
mate relation  with  it,  numerous  delicate 
vertical  bundles  of  non-striped  muscle, 
derived  from  the  underlying  muscularis 
mucosae,  ascend  towards  the  tip  of  the  villus. 

The  components  of  the  villus  are  held  together  by  the  common 
adenoid  tissue,  in  whose  interstices  lie  many  lymphoid  cells  and, 
during  certain  stages  of  digestion,  numberless  fatty  granules.     At 


fc=;  t3 


7 


Simple  tubular  glands  of  large 
intestine  of  dog:  the  epithelial 
elements  lining  the  follicles  have 
become  very  largely  converted 
into  goblet-cells. 


FlG.  207. 


fe:"^. 


Transverse  section  of  follicles 
of  large  intestine  of  dog  :  the 
individual  tubules  are  separated 
by  the  fibrous  stroma  of  the 
mucosa. 


170 


NORMAL   HISTOLOGY. 


such  times  the  contents  of  the 
Fig.  208. 


lacteals  appear  milky,  in  consequence 
of  the  emulsion  formed  by  the  ab- 
sorbed oil ;  during  the  intervals  of 
digestive  inactivity  the  lacteal  con- 
tains the  clear,  straw-colored  fluid 
usually  found  within  lymphatic  ves- 
sels. The  villi  disappear  abruptly 
at  the  ileo-caecal  valve  and  are  not 
present  in  the  large  intestine. 

Among  the  structures  of  the  in- 
testinal wall  usually  included  as 
"glands"  two  distinct  groups  must 


Fig.  209. 


Longitudinal  section  of  villus  from 
intestine  of  dog,  highly  magnified :  a, 
columnar  epithelium  containing  goblet- 
cells  (b)  and  migratory  leucocytes  (/z) ; 
c,  basement  membrane ;  d,  plate-like 
connective-tissue  elements  of  core;  e,  e, 
blood-vessels ;  f,  absorbent  radicle  or 
lacteal. 


Transverse  section  of  villus  from  intestine  of  dog  : 
a,  a,  blood-vessels  ;  b,  lacteal. 


be  recognized — the  true  and  the  false  glands,  the  latter  being  simple 
or  compound  lymph-follicles.  These  structures  therefore  fall  under 
the  appropriate  headings  : 


Intestinal  True-Glands. 

Glands  of  Lieberkiihn. 
Glands  of  B runner. 


Intestinal  Lymph-Follicles. 

Solitary  glands. 
Agminated  glands. 


The  follicles,  crypts,  or  glands  of  Lieberkiihn  are  very  nu- 
merous, forming  an  almost  continuous  layer  of  simple  tubular  de- 
pressions throughout  the  intestines,  large  as  well  as  small.  They 
occupy  nearly  the  whole  depth  of  the  mucosa,  their  wavy  extremities 
approaching  the  muscularis  mucosae.  The  columnar  epithelium  of 
the  free  surface  passes  directly  into  the  tubules  to  become  the  spherical 
secreting  cells,   many  of  which  undergo  mucoid  distention  and 


THE    DIGESTIVE   TRACT. 


171 


Lieberkiihn' s  glands  lie  between  the 


Fig.  210. 


Longitudinal  section  of  large  intestine  of  child  :  a, 
a,  simple  tubular  glands  ;  b,  submucous  tissue  ;  c  and 
d,  circular  and  longitudinal  layers  of  muscle. 


Fig.  211. 


conversion  into  goblet-cells, 
bases  of  the  villi,  but  are  found 
upon  the  valvulae  conniventes, 
since  the  latter  depend  on  the 
elevation  of  the  submucosa  for 
their  formation,  the  mucosa 
being  reflected  over  the  pro- 
jecting underlying  tunic.  In 
the  lower  part  of  the  large  in- 
testine the  glands  of  Lieber- 
kiihn  increase  in  size,  becom- 
ing longer  and  possessing 
wider  mouths,  their  orifices 
appearing  as  minute  pits. 

The  duodenum,  especially  in  its  upper  part,  possesses  an  additional 
layer  of  true  secreting 
structures  in  the  glands  of 
Brunner.  These  are  the 
direct  continuations  and 
higher  specializations  of 
the  pyloric  glands  of  the 
stomach.  In  passing  from 
the  stomach  into  the  intes- 
tine these  tubules  undergo 
repeated  division,  and,  at 
the  same  time,  sink  deeper 
and  deeper  into  the  mu- 
cosa, finally  reaching  below 
the  limits  of  this  layer  to 
take  up  a  position  within 
the  subtmicosa  of  the  duo- 
denum, beneath  the  over- 
lying layer  of  the  follicles 
of  Lieberkiihn  within  the 
mucosa. 

Brunner's  glands,  or 
the  duodenal  glands, 
appear  as  groups  of  short, 
wide,  tubular  acini,  dis- 
posed about  long,  slender 
ducts  which  pass  from  the  — «^^-^*is*-^^ss^i^^^  e> 

1                                              ,               ,  Section  of  duodenum  of  cat  :  a,  mucosa  containing  the 

SUDmUCOUS    tiSSUe    through  villi  (/)  and  the  follicles  of  Lieberkiihn  (i),  and  pierced  by 

the   mUCOSa  tO  Open  On  the  the  ducts  ^  of  the  Slands  of  Brunner  {h)  within  the  sub- 

;„i„   4.-       1             r            1  mucosa  (c) ;  i,  muscularis  mucosae  ;  d,  d',  circular  and  lon- 

lnteStinal    Surface     between  gitudinal  layers  of  muscle  ;  ,,  fibrous  tunic. 


172 


NORMAL   HISTOLOGY. 


Fig.  212. 


the  orifices  of  the  follicles  in  the  depressions  between  the  bases  of  the 
surrounding  villi.  The  glands,  owing  to  the  rapid  branching  of  their 
tubules,  more  closely  approach  the  racemose  type  than  the  compound 

tubular  to  which  they 
really  belong,  as  shown 
in  their  direct  deriva- 
tion from  the  com- 
pound tubular  pyloric 
crypts.  The  secretion 
of  these  duodenal 
glands  is  serous  and 
not  mucous,  the  cells 
being  filled  with  dark 
granules. 

The       solitary 
glands     are     isolated 
lymph  -  follicles      scat- 
Section  of  human  large  intestine,  containing  solitary  gland:  a,       tered         thrOU°'h         the 
mucosa  ;  b,  submucosa  ;  c,  c',  circular  and  longitudinal  layers  of  .  .  . 

muscle;  d,  serous  coat.  entire   intestine;    they 

are,  however,  most 
abundant  in  the  lower  part  of  the  ileum  and  in  the  first  portions  of  the 
large  intestine.  They  are  situated  primarily  within  the  mucosa,  al- 
though they  frequently  lie  also  within  the  submucous  coat ;  when  well 


Section  of  small  intestine  of  cat,  showing  a  Peyer's  patch  (d,  d)  cut  crosswise  :  a,  b,  c,  respectively 
mucous,  submucous,  and  muscular  coats. 


developed,  they  encroach  upon  the  mucosa  to  such  an  extent  that  their 
inner  pole  slightly  projects  upon  the  free  surface  of  the  intestine.  The 
lymphoid  tissue  is  somewhat  denser  in  the  periphery  of  the  fol- 
licle, beneath  its  limiting  capsule,  than  towards  the  centre ;  but  the 


THE    DIGESTIVE    TRACT.  ly-, 

lymphoid  cells  are  even-where  so  closely  packed  that  the  support- 
ing reticulum  of  connective  tissue  is  masked.  In  the  upper  part  of 
the  duodenum  numerous  ill-defined  masses  of  adenoid  tissue  occupy 
the  mucosa  between  the  follicles  and  represent  the  lenticular  glands 
of  the  stomach. 

The  agminated  glands,  or  Peyer's  patches,  are  large,  oval 
groups  of  closely  aggregated  lymph-follicles,  held  and  blended  to- 
gether by  diffuse  adenoid  tissue.  These  patches  vary  in  size  and 
number,  and  are  usually  limited  to  the  lower  two-thirds  of  the  small 
intestine,  reaching  their  highest  development  in  the  ileum,  where 
they  may  attain  a  length  of  9-1 1  cm.  ;  between  twenty  and  thirty 
patches  generally  are  present,  while  they  are  relatively  better  devel- 
oped in  young  than  in  old  subjects. 

The  agminated  glands  appear  first  within  the  mucosa,  but  later 
encroach  largely  upon  the  submucous  tissue.  The  lymph-follicles 
of  which  these  patches  are  composed  become  somewhat  pyramidal, 
owing  to  pressure,  and  lose  much  of  their  individuality,  the  demarca- 
tion  into  separate  follicles  being  best  preserved  along  the  outer 

Fig.  214. 


wM\  ■  »  ^c 


&%■ 


Section  of  small  intestine  of  child,  including  a  portion  of  a  Peyer's  patch  :  a.  5,  and  c,  mucosa,  sub- 
mucosa,  and  muscular  coats ;  d,  villi;  e,  e,  atrophic  follicles  of  the  mucosa. 

boundary,  occupying  the  submucosa,  within  the  mucosa  the  out- 
lines of  the  follicles  being  lost  in  the  general  adenoid  mass.  Where 
the  summits  of  the  follicles  impinge  against  the  inner  layer  of  the 
mucosa,  the  positions  of  the  follicles  are  indicated  by  corresponding 
elevations  of  the  mucous  surface,  at  which  points  the  villi  are  fre- 
quently pushed  aside  and  the  gland-layer  more  or  less  completely 
interrupted.     In  the  vermiform  appendix  of  some  animals,   and 


yja  NORMAL   HISTOLOGY. 

in  some  cases  also  in  man,  the  follicles  form  a  continuous  zone  of 
adenoid  tissue. 

The  muscularis  mucosae,  like  that  of  the  stomach,  occupies  the 
deepest  part  of  the  mucosa  and  marks  the  outer  boundary  of  the 
mucous  layer.  The  muscular  tissue  comprises  longitudinally-dis- 
posed bundles  of  muscle-cells,  supplemented  in  some  places  by  a 
more  or  less  complete  additional  internal  layer  of  circularly-placed 
cells. 

The  submucosa  of  the  intestinal  wall  corresponds  to  the  similar 
coat  of  the  stomach,   consisting  of  loosely-united  bundles  of  fibro- 

elastic  tissue,  which  support 
the  larger  vascular  and  lym- 
phatic trunks,  as  well  as  a  rich 
nervous  plexus. 

The  muscular  coat  con- 
sists of  two  well-developed 
layers — the  thicker  inner  cir- 
cular and  the  less  robust 
outer  longitudinal  stratum. 
These  are  separated  by  a  thin 
layer  of  connective  tissue, 
which  externally  becomes 
continuous  with  the  envel- 
oping areolar  tissue  and  passes 
into  the  outer  fibrous  tunic  of 
the  serosa. 

In  parts  of  the  large  intes- 
tine— as  the  caecum  and  the 
colon — the  circular  muscular 
coat  is  relatively  thin,  while 
the  longitudinal  layer  is  in- 
complete, the  fibres  of  the  lat- 
ter being  collected  into  three 
flat  bands,  10—15  mm.  wide  ; 
these  longitudinal  bands  are 
much  shorter  than  the  other 
layers  of  the  intestinal  wall, 
which  arrangement  results  in 
the  characteristic  sacculation 
of  the  large  intestine.  In  the 
lower  part  of  the  rectum  the  circular  muscular  layer  becomes  thick- 
ened to  form  the  internal  anal  sphincter,  composed  of  involuntary 
muscle ;  the  bands  of  longitudinal  fibres  spread  out,  and  towards  the 
lower  end  of  the  rectum  form  a  thick,  uniform  layer. 


Section  of  injected  small  intestine  of  cat  :  a,  b,  mu- 
cosa;  g,  villi;  i,  their  absorbent  vessels;  h,  simple 
follicles  ;  c,  muscularis  mucosa;  ;  d,  submucosa  ;  e,  e' , 
circular  and  longitudinal  layers  of  muscle  ;  /,  fibrous 
coat.  All  the  dark  lines  represent  blood-vessels  filled 
with  the  injection  mass. 


THE   DIGESTIVE   TRACT.  1j* 

The  blood-vessels  supplying  the  intestines  follow  the  general 
arrangement  of  those  of  the  stomach.  The  larger  vessels  pierce  the 
serous  and  muscular  coats,  giving  off  slender  twigs  to  supply  the 
tissues  of  these  tunics ;  upon  reaching  the  submucosa  the  vessels 
form  a  wide-meshed  net-work.  Numerous  branches  then  pass 
through  the  muscularis  mucosae  to  be  distributed  to  the  deeper  as 
well  as  to  the  more  superficial  parts  of  the  mucosa;  narrow  capil- 
laries form  net-works  which  surround  the  tubular  glands,  while  be- 
neath the  epithelium  wider  capillaries  encircle  the  mouths  of  the 
follicles.  From  this  superficial  capillary  net-work  the  veins  arise  and, 
passing  between  the  follicles,  join  the  deeper  venous  plexus,  which 
in  turn  empties  into  the  larger  veins  of  the  submucosa. 

In  those  parts  of  the  intestine  where  villi  exist,  special  additional 
arteries  pass  directly  to  the  bases  of  the  villi,  where  they  expand  into 
capillary  net-works  which  run  beneath  the  epithelium  and  around 
the  central  lacteal  as  far  as  the  ends  of  the  villi.  These  capillaries 
terminate  in  venous  stems  which  descend  almost  perpendicularly 
into  the  mucosa,  in  their  course  receiving  the  superficial  capillaries 
encircling  the  glandular  ducts.  Brunner's  glands  and  the  solitary 
and  agminated  follicles  are  supplied  from  the  submucosa  by  vessels 
which  terminate  in  capillary  net-works  distributed  to  the  acini  of  the 
glands  and  to  the  interior  of  the  lymph-follicles. 

The  lymphatics  of  the  intestinal  tract  are  very  abundant.  They 
begin  as  blind  canals,  whose  slightly-dilated  ends  lie  within  the  mu- 
cosa between  the  tubular  follicles  ;  in  those  parts  of  the  intestine 
where  villi  exist,  the  centre  of  these  projections  is  occupied  by  a 
lymphatic  radicle,  the  chyle-vessel,  or  lacteal.  All  these  vessels  de- 
scend to  join  a  rich  plexus  of  lymphatic  trunks  situated  within  the 
deeper  layers  of  the  mucosa.  Within  the  submucosa  an  addi- 
tional net-work  of  still  larger  channels  exists,  the  two  sets  of  vessels 
freely  communicating  through  numerous  anastomoses.  The  accumu- 
lations within  these  net-works  are  carried  off  by  lymphatic  trunks 
which  pierce  the  muscle  and  pass  off  between  the  two  layers  of  the 
peritoneum  into  the  adjacent  mesenteric  glands,  in  their  course 
taking  up  the  vessels  carrying  the  lymph  collected  from  the  mus- 
cular tissue.  Many  vessels  of  the  submucous  net-work,  as  well  as 
the  larger  lymphatic  trunks,  are  provided  with  valves,  whose  position 
is  usually  indicated  by  dilatations  in  the  contour  of  the  vessel. 

The  nerves  distributed  to  the  intestines  are  arranged  almost  iden- 
tically as  those  of  the  stomach ;  they  are  composed  largely  of  non- 
medullated  fibres,  derived  from  the  trunks  which  pass  within  the 
mesentery  from  the  large  abdominal  sympathetic  plexuses.  After 
giving  off  branches  to  the  serous  coat,  the  nerves  pierce  the  longitu- 
dinal muscular  tunic  to  form  the  rich  intramuscular  plexus  of  Auer- 


I7g  NORMAL   HISTOLOGY. 

bach.  This  is  composed  of  a  rich  net-work  of  delicate,  pale  fibres, 
at  the  nodal  points  of  which  microscopic  ganglia  exist ;  after  supply- 
ing the  longitudinal  and  outer  part  of  the  circular  muscular  coats,  the 
fibres  obliquely  pierce  the  latter  tunic  to  gain  the  submucous  tissue, 
where  they  form  the  plexus  of  Meissner,  which  closely  resembles 
Auerbach's  nervous  net-work  within  the  muscularis,  possessing,  how- 
ever, smaller  ganglia  and  somewhat  closer  meshes.  From  the  plexus 
of  the  submucous  tunic  fibres  pass  into  the  mucosa  to  form  net-works 
about  the  glands  and  to  send  fibrillae  into  the  villi.  The  ultimate 
distribution  of  these  fibres  must  be  regarded  as  still  undetermined. 


THE    LIVER. 

Although  the  liver  in  its  development  corresponds  to  a  compound 
tubular  gland,  a  type  which  is  permanently  retained  in  many  lower 

vertebrates,  in  the  adult 
Fig.  216.  condition  of  the  mam- 

malian organ  this  char- 
acter is  largely  masked 
in  consequence  of  the 
fusion  of  the  tubes  in 
the  formation  of  the 
cords  of  cells. 

The  fibrous  tissue 
enveloping  the  exterior 
of  the  liver  is  prolonged 
into  the  interior  of  the 
organ      through      the 
transverse    fissure, 
company     with 
blood-vessels    and 
bile  -  ducts.      The 


v& 


te?a. 


Section  of  liver  of  hog,  showing  very  diagram matically  the 
lobules  :  a,  interlobular  connective  tissue;  b,  c,  branches  of  por- 
tal vein  and  of  hepatic  artery  ;  d,  bile-ducts ;  e,  intralobular  vein. 


in 
the 
the 
de- 


marcation of  the  indi- 
vidual lobules  depends  upon  the  development  of  this  interlobular 
connective  tissue,  known  as  the  capsule  of  Glisson ;  when  well 
developed,  as  in  the  liver  of  the  hog,  the  lobules  are  defined  with 
great  distinctness,  being  completely  surrounded  and  separated  from 
their  neighbors  by  the  connective  tissue.  In  the  human  liver,  on  the 
contrary,  the  interlobular  connective  tissue  is  very  scanty,  this  defici- 
ency producing  poorly-defined  lobules,  the  boundaries  of  which  are 
scarcely  indicated  by  the  irregular  areas  of  connective  tissue  occupy- 
ing the  spaces  between  the  approximated  surfaces  of  three  or  more 
hepatic  lobules. 

The  arrangement  of  the  blood-vessels  is  so  important  in  de- 
termining the  general  construction  of  the  lobule  that  an  early  con- 


THE   DIGESTIVE   TRACT. 


177 


Fig.  218. 


Section  of  human  liver,  showing  general  arrangement  of  lobules  :  a,  interlobular  (portal)  vein ;  b, 
intralobular  (hepatic)  vein  ;  c,  hepatic  artery  ;  d,  bile-duct ;  the  boundaries  of  the  lobules  are  imper- 
fectly defined  by  the  irregular  areas  representing  the  poorly-developed  capsule  of  Glisson. 

sideration  of  the  vascular  supply  is  necessary  to  an  understanding  of 
the  structure  of  the  lobule. 

The  interlobular  vessels,  situated 
between  the  lobules  at  their  periphery, 
are  continuations  of  those  passing 
through  the  transverse  fissure ;  they 
are  the  portal  vein,  the  hepatic  ar- 
tery, and  the  bile-duct. 

The  portal  vein,  the  largest  of 
the  interlobular  vessels,  gives  off  nu- 
merous branches,  which  enter  the 
lobule  at  the  periphery  and  break  up 
into  twigs,  forming  a  rich,  freely  anas- 
tomosing intralobular  capillary 
net-work.  The  meshes  of  this  net- 
work are  somewhat  elongated  and 
trapezoidal  in  form,  the  smaller  end 
of  the  spaces  being  directed  towards 
the  centre  of  the  lobule,  an  arrange- 
ment produced  by  the  convergence  of 
the  capillary  net-work  to  the  centrally 
placed  intralobular  vein,  a  branch 
of  the  hepatic. 

The  meshes  of  this  lobular  capil- 
lary  net-work  are   occupied   by   the 

12 


Diagram  of  the  structure  of  the  liver : 
P.  V.,  the  portal  or  interlobular  vein, 
which  breaks  up  into  the  capillary  net-work 
of  the  lobule ;  H.  V.,  central  intralobular 
vein,  a  branch  of  the  hepatic;  H.  A.,  he- 
patic artery,  supplying  nutrition  to  the  in- 
terlobular structures  and  terminating  in  the 
lobular  capillar}'  net-work  ;  B.D.,  the  inter- 
lobular bile-duct  which  takes  up  the  bile- 
capillaries  at  the  periphery  of  the  lobule. 


j  ^3  NORMAL    HISTOLOGY. 

secreting  hepatic  tissue,  comprising  the  liver-cells,  the  bile-capil- 
laries, the  minute  channels  through  which  the  bile  elaborated  within 

Fig.  219. 


Section  of  injected  human  liver,  the  capillaries  having  been  filled  from  the  central  vein  (a)  ; 
b,  branches  of  portal  vein. 


the  lobule  is  carried  off,  together  with  lymph-radicles  and  a  very- 
small  amount  of  delicate  areolar  tissue.  The  liver-cells  are  irreg- 
ular   polyhedral    elements  (17- 

25  (j.)  in  whose   finely  granular  Fig.  221. 

protoplasm,  devoid  of  cell-mem- 
brane, one  or  more  round  nuclei 
lie  embedded.  Numerous  oil- 
drops  of  various  sizes,  as  well  as 
pigment  -  granules,  very  com- 
monly  are    present   within    the 

Fig.  220. 


Hepatic  cells  isolated  from  human  liver  :  a, 
oil-drops ;  b,  slight  concavity  produced  by 
blood-vessels. 


Section  of  uninjected   human  liver:   a,  cords  of 
liver-cells  lying  between  the  blood-channels  (b). 


protoplasm.      The  variations  in  the  apparent  granularity  of  the  cells 
depend,  as  in  other  glands,  upon  the  condition  of  functional  activity  : 


Section  of  centre  of  lobule  of 
human  liver:  a,  intralobular 
vein,  into  which  the  capillaries 
{6)  converge  ;  c,  hepatic  tissue. 


THE  DIGESTIVE  TRACT.  l?Q 

the    nearer    complete    exhaustion,    the    more    emphasized    are    the 
granules. 

The  meshes  of  the  capillary  net-work  are  usually  only  suffi- 
ciently wide  to  accommodate  a  few  liver-cells,  in  consequence  of 
which  arrangement  almost  every  hepatic  ele- 
ment is  bounded  directly  on  at  least  one  side 
by  a  capillary  blood-vessel,  a  relation  con- 
ducive to  free  interchange  between  the  blood 
and  protoplasm  of  the  cells.  With  few  excep- 
tions every  liver-cell  exhibits  a  slight  con- 
cavity on  one  border,  which  denotes  the 
position  of  contact  and  impression  by  the 
blood-vessels. 

In  uninfected  organs  the  hepatic  tissue  ap- 
pears made  up  of  irregular,  branching,  and 
anastomosing  cords  of  cells,  which  form 
close  net-works,  the  intervening  clear  clefts 
being  the  lumina  of  the  blood-capillaries. 
According  to  Disse's  studies,  the  liver-cells 
do  not  lie  immediately  in  contact  with  the 

capillaries,  but  are  separated  from  the  latter  by  delicate  perivascu- 
lar lymphatic  channels  which  envelop  the  blood-capillaries. 

In  livers  still  retaining  their  primitive  type  of  the  tubular  gland 
the  bile-capillaries  appear  as  minute 
ducts  placed  centrally  within  the  cords 
of  the  hepatic  cells,  the  biliary  passages 
representing  lumina  of  tubular  acini 
lined  with  secreting  glandular  epithe- 
lium. In  man,  however,  the  liver-cells 
are  usually  bordered  on  all  sides,  ex- 
cept on  that  lying  next  the  blood- 
vessels, by  the  delicatebile-canaliculi, 
the  latter  never  interposing  between 
the  cells  and  the  blood-channels. 

The  bile- capillaries  exist  as  narrow 
(1-2  fi)  clefts  between  adjacent  liver- 
cells,  maintaining  about  the  same 
diameter  throughout  the  lobule  ;  at  the  periphery  the  intercellular 
channels  pass  into  the  larger,  though  still  small,  interlobular  bile- 
ducts.  The  hepatic  cells  between  which  the  bile-capillary  takes  its 
course  become  replaced  at  the  periphery  of  the  lobule  by  the  low 
epithelium  of  the  bile-duct,  the  basement-membrane  present  in  the 
latter  fading  away  into  the  delicate  connective  tissue  holding  together 
the  cords  of  liver-cells. 


Fig.  22^. 


Section  of  liver  of  frog,  exhibiting  tubu- 
lar character  of  gland  :  a,  blood-channels 
containing  corpuscles  ;  i,  lumina  of  hepatic 
cylinders  which  correspond  to  bile-capil- 
laries ;  c,  pigment-cell. 


I  So 


NORMAL    HISTOLOGY. 


Section  of  rabbit's  liver  in  which  the  bile-capillaries 
(b)  have  been  injected  and  appear  as  dark  lines  between 
the  cells  :  c,  blood-channels. 


The  existence  of  a  distinct  independent  wall  to  the  bile-capillaries 
has  been  the  subject  of  much  conflicting  testimony;  according  to 

some,  these  vessels  are  with- 

Fig.  224.  out    distinct    walls    of    their 

a,  own,   while    other  authorities 

regard  the  existence  of  a  deli- 
cate special  wall  consisting  of 
a  homogeneous   structureless 
membrane      as      established. 
The    presence   of   a    distinct 
membranous  wall  seems  ques- 
tionable ;   when  it  is  recalled 
that  the  bile-capillaries  really 
represent    lumina    of   modi- 
fied   tubular    glands,  there 
seems  to  be  no  greater  neces- 
sity for  or  probability  of  the 
existence  of  a  membrane   to 
limit   the  lumen  of  the   bile- 
tubule  than  in  the  case  of  other  glands.      The  direct  transforma- 
tion of  the  secreting  hepatic  cells  into  the  epithelium  of  the  bile- 
duct   at   the   margin   of   the 
Fig.  225.  lobule    further    opposes    the 

assumption  of  such  limiting 
membrane,  while  examination 
of  livers  in  which  the  tubu- 
lar type  of  the  acini  is  re- 
tained fails  to  show  such 
structures  within  the  lumina 
of  the  tubes. 

Emerging  from  the  lobule 
at  the  periphery  to  pass  into 
the  adjacent  interlobular  con- 
nective tissue,  the  small  bile- 
ducts  empty  into  the  larger 
ones,  which  bear  the  branches 
of  the  hepatic  blood-vessels 
company.  The  interlobular 
bile-vessels  gradually  in- 
crease in  size,  owing  to  the 
repeated  union  of  the  smaller  tubes,  until  the  larger  trunks  unite  to 
form  the  hepatic  duct.  While  the  walls  of  the  smaller  bile-ducts 
consist  of  columnar  epithelium  strengthened  by  fibrous  con- 
nective tissue  mixed  with  elastic  fibres,  those  of  the  large  vessels 


cl 


Section  of  liver  of  dog,  including  portion  of  lobule  and 
interlobular  connective  tissue  {a) ;  b,  portal  vein  ;  c, 
hepatic  artery  ;  d,  bile-ducts  ;  e,  small  peripheral  bile- 
vessel  ;  g,  blood-channels  ;  h,  hepatic  tissue. 


THE  DIGESTIVE  TRACT. 


181 


Fig.  226. 


Transverse  section  of  large  bile-duct 
from  human  liver :  a,  epithelial  lining; 
b,  fibro-muscular  coat ;  c,  surrounding 
areolar  tissue. 


comprise  an  outer  fibrous  adventitia  and  an  inner  mucous 
membrane.  The  latter,  in  addition  to  the  columnar  epithelium, 
consists  of  the  tunica  propria,  containing 
many  elastic  fibres  and  some  delicate 
bundles  of  involuntary  muscle,  irregu- 
larly disposed  as  circular  and  longitudi- 
nal bundles.  Small  mucous  glands  also 
occur  within  the  mucosa  of  the  larger 
canals  and  of  the  hepatic  duct.  The  inter- 
lobular bile-ducts  may  be  distinguished 
from  blood-vessels  of  the  same  size  by 
their  lining  of  columnar  epithelium. 

The  blood-vessels  of  the  liver,  as 
already  described,  are  of  primary  impor- 
tance in  determining  the  arrangement 
of  the  hepatic  tissue.  The  blood  brought 
by  the  interlobular  branches  of  the  portal 

vein  passes  into  the  lobule  at  the  periphery  by  the  numerous  twigs  ; 
these,  on  entering  the  lobule,  form  a  closely  anastomosing  intra- 
lobular capillary  net-work,  which  converges  to  a  central  intra- 
lobular vein.  The  central  vessel  is  vertically  placed  with  regard  to 
the  general  plane  of  the  capillary  net-work,  and  empties  into  the  ad- 
jacent sublobular  veins,  which  are  branches  of  the  hepatic  vein, 
lying  within  planes  generally  at  right  angles  to  those  of  the  portal 
vessels. 

The  hepatic  artery  has  directly  nothing  to  do  with  the  elabora- 
tion of  the  especial  products  of  the  organ,  its  particular  province 
being  to  supply  the  blood  for  the  nutrition  of  Glisson's  capsule 
and  of  the  interlobular  structures,  including  the  blood-vessels  and 
the  bile-ducts.  Minute  arterial  twigs  are  distributed  to  the  walls  of 
these  tubes,  where  they  end  in  delicate  capillary  net-works,  which, 
in  turn,  at  the  periphery  of  the  lobule,  pour  their  contents  into  the 
intralobular  net-work  of  the  portal  vein. 

The  lymphatics  of  the  liver  constitute  a  superficial  and  a  deep 
system.  The  superficial  lymphatics  accompany  the  branches  of 
the  arteries  supplying  the  capsule,  and  form  a  close-meshed  sub- 
serous net-work  within  the  capsule. 

The  interlobular  blood-vessels  are  accompanied  by  numerous  lym- 
phatics, whose  ramifications  and  anastomoses  constitute  the  deeper 
plexus.  The  presence  of  lymphatics  within  the  parenchyma  of  the 
liver  is  still  a  matter  of  dispute.  According  to  Disse,  the  lym- 
phatic channels  exist  throughout  the  lobule  as  perivascular  canals, 
surrounding  the  capillaries  and  separating  them  from  direct  contact 
with  the  secreting  cells. 


X82  NORMAL   HISTOLOGY. 

The  main  nerve-trunks  of  the  liver  enter  at  the  transverse  fissure 
in  company  with  the  blood-vessels  and  the  lymphatics.  The  fibres 
consist  largely  of  the  non-medullated,  together  with  a  smaller  number 
of  the  medullated  variety.  These  nerves  run  within  the  interlobular 
connective  tissue  in  company  with  the  hepatic  artery.  They  may 
be  traced  with  certainty  to  the  periphery  of  the  lobule ;  regarding 
the  exact  mode  of  their  ultimate  distribution,  however,  nothing  is 
definitely  known.  Minute  ganglia  occur  along  the  interlobular 
trunks. 

The  gall-bladder,  or  bile-sac,  possesses  walls  composed  essen- 
tially of  the  same  tissues  as  those  of  the  larger  bile-ducts,  these  consist- 
ing of  a  mucous  membrane  supplemented  by  oblique  bands  of  invol- 
untary muscle  and  an  outer  fibrous  coat.  The  mucosa  is  thrown 
into  minute  folds  or  rugae,  which  unite  and  interlace  to  form  a  net- 
work of  ridges  and  give  to  the  surface  of  the  mucous  membrane  a 
reticulated  appearance. 

The  blood-vessels,  the  lymphatics,  and  the  nerves  form  net-works 
within  the  mucosa,  which  usually  terminate  in  the  superficial  or  inner 
layers  of  the  tunica  propria. 

THE   ACCESSORY    DIGESTIVE    GLANDS. 

These  include  the  salivary  glands — the  parotid,  the  submaxillary, 
and  the  sublingual — and  the  pancreas.  While  in  their  quiescent, 
immature  condition  all  are  similar,  after  full  functional  development 
is  attained  the  variation  in  the  character  of  their  secretions  leads  to 
the  recognition  of  two  groups — the  serous  and  the  mucous  sali- 
vary glands.  Those  of  the  serous  type,  regarded  as  the  true  sali- 
vary glands,  are  represented  in  man  and  mammals  by  the  parotid 
gland  and  the  pancreas.  The  mucous  glands  are  best  represented 
in  man  and  many  animals  by  the  sublingual,  although  the  presence 
of  serous  acini  places  this  organ,  strictly  considered,  within  the  cate- 
gory of  the  mixed  glands. 

The  muco-serous  or  mixed  glands  are  exemplified  by  the  sub- 
maxillary of  man  and  many  mammals  (as  apes,  guinea-pig,  etc.); 
in  other  animals  (as  dog  or  cat)  this  gland  is  entirely  mucous,  while 
in  certain  others  (as  the  rabbit)  it  is  a  true  serous  gland. 

THE   SALIVARY    GLANDS. 

The  parotid  is  a  compound  saccular  or  racemose  gland,  en- 
veloped in  a  general  fibrous  capsule  from  which  stout  connective- 
tissue  septa  penetrate  the  organ,  dividing  the  gland  into  lobes. 
These  latter  are  subdivided  by  fibrous  partitions  into  numerous  lob- 
ules, each  of  which,  in  turn,  is  composed  of  groups  of  the  ultimate 
saccules  or  acini. 


THE    DIGESTIVE   TRACT.  jg. 

The  large  excretory  duct  of  the  parotid  gland,  or  Stenson's 
duct,  contained  within  the  interlobular  connective  tissue,  is  composed 
of   a    fibro-elastic    tunica 
propria,  lined  by  a  simple  Fig 

low  columnar  epithe-  MHSwasMBaBSa 
lium,  and  strengthened 
externally  by  fibrous  tissue. 
Passing  into  the  smaller 
ducts,  the  salivary  tubes, 
the  cylindrical  epithelium 
becomes  slightly  taller,  and 
exhibits  a  distinct  vertical 
radial  striation  in  its  outer 
zone.  On  entering  the 
intralobular  divisions  of 
the  ducts,  or  interme- 
diate tubes,  the  columnar 
epithelium  is  replaced  by 
low  flattened  cells,  which 
finally  pass  into  the  dilated 
terminal  compartments,  be- 
coming directly  continuous  with  the  secreting  cells  lining  the  acini. 

The  acini  are  limited  by  the  basement-membrane,  the  prolonga- 
tion of  that  of  the  smaller  ducts,  and  almost 
completely  filled  by  the  irregularly  polyhedral 
glandular  epithelium,  the  narrow  intercellular 
cleft  which  remains  representing  the  commence- 
ment of  the  lumen  of  the  system  of  ducts. 
The  appearance  of  the  cells  of  the  acini  varies 
with  the  stages  of  secretion ;  when  quies- 
cent and  filled  with  the  serous  secretion,  the 
cells  appear  larger,  clearer,  and  less  granular, 
while  after  functional  activity  and  in  the  ex- 
hausted condition  they  are  smaller,  darker,  and 
more  granular,  the  granules  of  the  protoplasm 
lying  closely  packed,  and  not,  as  when  the  gland  is  at  rest 
rated  by  the  intervening  particles  of  stored-up  secretion. 

The  sublingual  gland  possesses  the  general  arrangement  already 
considered  in  connection  with  the  parotid  gland,  its  peculiarity  being 
the  absence  of  the  intermediate  division  of  the  duct,  the  intralobular 
or  ' '  mucous' '  tubes  passing  at  once  into  the  acini. 

The  cells  lining  the  saccules  are  encountered  in  all  stages  of  secre- 
tion. During  rest  the  majority  are  clear,  being  filled  with  homo- 
geneous viscid  mucus.      After  the  discharge  of  this  secretion,  fol- 


mm 

fr  a  a 

Section  of  human  parotid  gland,  exhibiting  general  ar- 
rangement of  lobules  (a)  ;  b,  interlobular  connective  tissue 
containing  large  ducts  (c)  and  blood-vessels  (v)  ;  d,  intra- 
lobular ducts. 


Fig.  228. 


Section  of  human  parotid 
gland,  including  several 
acini :  d,  cut  intralobular 
duct. 


sepa- 


1$a  NORMAL    HISTOLOGY. 

lowing  prolonged  activity,  the  cells  appear  smaller,  dark  and 
granular,  and  closely  resemble  the  elements  of  the  serous  glands, 
since  the  mucoid  substance  separating  the  particles  of  the  cell  pro- 
toplasm has  been  removed,  thereby  allowing  the  displaced  proto- 
plasmic granules  once  more  to  approach  closely. 

Not  all  the  cells  in  the  resting  acini  are  in  the  same  secretory 
condition,  since  quite  usually  certain  cells  have  failed  to  participate 
in   the  activity  of  their   neighbors,  and,   in 
Fig.  229.  consequence,  appear  as  crescentic  groups 

of  granular  cells  lying  immediately  next  the 
basement-membrane  at  the  periphery  of  the 
acinus,  where  they  have  been  crowded  by  the 
larger  mucus-filled  elements.  These  cres- 
centic groups  constitute  the  demilunes  of 
Heidenhain  or  the  crescents  of  Gia- 
nuzzi,  and  are  aggregations  of  cells  which 
Section  of  human  sublingual     have  not  participated  in  secretion.     The  ex- 

gland :    among   the   clear  cells 

lining  the  mucous  acini  are  cretory  tube  of  the  sublingual  gland,  or  the 
nests  or,  g)  of  granular  eie-     duct  of  Bartholin,  consists  principally  of  a 

ments     which     constitute     the        ~,  .         .  .  .  .,.,.,. 

demilunes  of  Heidenhain.  nbro-elastic  tunica  propria,  within  which  is  a 

single  layer  of  low  columnar  cells,  while  out- 
side extends  a  supplementary  layer  of  fibrous  tissue. 

The  submaxillary  gland  is  a  mixed  gland,  certain  lobules  being 
composed  of  acini  of  the  serous  type,  while  neighboring  divisions 
contain  those  of  the  mucous  variety. 

The  excretory  channel,  or  the  duct  of  Wharton,  resembles  that 
of  the  parotid  gland,  dividing  into  the  smaller  tubes  lined  by  striated 
"rod"  epithelium,  passing  thence  into  the  intermediate  tubules,  with 
low  cuboidal  cells,  which  lead  into  the  serous  acini  filled  with  dark 
granular  cells  on  the  one  hand,  or  into  those  filled  with  mucous  cells 
and  granular  crescents  on  the  other. 

The  vascular  supply  of  the  salivary  glands  is  very  rich  ;  while 
the  arrangement  of  the  blood-vessels  in  the  several  glands  presents 
unimportant  differences,  their  distribution  is  according  to  the  same 
general  plan. 

The  larger  arteries  accompany  the  excretory  ducts  of  the  glands 
within  the  interlobular  fibrous  septa,  where  they  give  off  branches 
which  pass  between  the  lobules  and  later  penetrate  the  tissue  of  the 
lobules  to  end  in  rich  capillary  net-works  enclosing  the  acini.  The 
capillaries  lie  immediately  outside  the  basement-membrane  in  prox- 
imity to  the  secreting  cells.  The  veins  follow  the  general  course 
taken  by  the  arteries. 

The  lymphatics  are  represented  by  indefinite  interfascicular  clefts 
between  the  acini,  which  are  taken  up  by  definite  lymph-vessels 


THE   DIGESTIVE   TRACT.  jge 

situated  within  the  interlobular  connective  tissue,  the  larger  trunks 
accompanying  the  main  blood-vessels. 

The  nerves  distributed  to  the  salivary  glands  constitute  a  rich 
supply,  composed  of  both  medullated  and  pale  fibres.  From  the 
larger  trunks  of  the  interlobular  net-works,  along  the  course  of  which 
minute  ganglia  occur,  smaller  branches  enter  the  lobules  and  extend 
between  the  acini.  Regarding  the  ultimate  distribution  of  the  many 
fibres  passing  to  the  glandular  tissue  little  is  definitely  known,  not- 
withstanding the  laborious  investigations  undertaken  with  a  view 
to  solve  this  difficult  problem.  The  nerve-fibres  may  be  traced  to 
the  basement-membrane  of  the  acini,  around  which  net-works  are 
formed  ;  as  to  the  further  fate  of  the  fibrillae,  however,  little  can  be 
regarded  as  proved.  While  an  intimate  relation  between  the  nerves 
and  the  secreting  cells  may  be  assumed  as  undoubtedly  existing,  no 
direct  continuity  between  these  structures  has  been  established,  not- 
withstanding the  already-published  assertions  and  elaborate  descrip- 
tions of  such  connections. 


Fig 


THE    PANCREAS. 

The  pancreas  is,  as  aptly  described  by  its  German  name,  "  Bauch- 
speicheldriise,"  the  abdominal  salivary  gland,  belonging  to  the 
serous   type,   and  closely  corresponding 
in  structure  and  in  the  nature  of  its  secre- 
tion to  the  parotid  gland. 

The  connective-tissue  framework  of 
the  organ  divides  the  glandular  tissue  into 
lobes,  which  are  subdivided  by  septa  into 
the  lobules,  these,  in  turn,  being  com- 
posed of  groups  of  acini.  The  laminated 
fibrous  connective  tissue  constituting  the 
walls  of  the  pancreatic  duct  is  clothed  by 
a  single  layer  of  columnar  epithelium. 
The  branches  of  the  main  duct  divide  at 
once  into  the  long  intermediate  tubules, 
the  intralobular  ducts,  or  salivary  tubes, 
being  wanting  ;  it  follows  that  the  vertical 

striation  of  the  epithelium  lining  these  tubes,  so  conspicuous  in  sections 
of  the  parotid  gland,  is  absent  in  the  pancreas. 

The  cylindrical  cells  of  the  larger  ducts  gradually  pass  into  the 
lower  cuboidal  and  flattened  plates  lining  the  intermediate  tubules. 
The  acini  of  the  pancreas  are  more  tubular  than  those  of  the  parotid 
gland,  while  the  secreting  cells  suggest  more  strongly  the  cylin- 
drical or  pyramidal  type  than  those  of  the  salivary  gland  ;  these 
cells  are  further  characterized  by  the  presence  of  a  zone,  next  the 


Section  of  human  pancreas,  in- 
cluding several  acini  and  two  ducts  : 
the  cells  present  a  central  granular 
and  a  peripheral  clear  zone. 


1 86 


NORMAL   HISTOLOGY. 


Fig.  231. 


lumen  of  the  acinus,  containing  numbers  of  highly  refracting  par- 
ticles, while  the  peripheral  outer  half  of  the  cells  contains  the  nucleus 
and  is  comparatively  free  from  the  granules.  The  relations,  how- 
ever, between  the  clear  and  granular  zone  of  the  pancreatic  cells  are 
not  constant,  but  vary  with  the  condition  of  functional  activity. 
During  the  earliest  stages  of  digestion,  when  the  cells  are  filled 
with  secretion,  the  clear  zone  occupies  almost  the  entire  cell,  the 
granules  being  confined  to  a  narrow  belt  immediately  around  the 
lumen ;  towards  the  close  of  a  period  of  functional  activity,  on  the 
contrary,  the  granules  occupy  the  greater  part  of  the  cell,  while 
the  clear  zone  is  reduced  to  a  narrow  peripheral  area  ;  during  fasting 
the  clear  and  the  granular  zone  about  equally  divide  the  cells. 

On  examining  sections  of  pancreas  under  low  amplification,  certain 
round  or  oval  areas  appear  lighter  and  less  dense  than  the  ordinary 

tissue  of  the  organ.  These  peculiar 
areas,  or  bodies  of  Langerhans, 
under  high  magnification  prove  to 
be  composed  of  groups  of  small, 
imperfectly-developed  acini,  among 
and  about  which  ramify  rich  capil- 
lary net-works,  whose  frequently 
tortuous  course  and  lobulated  ar- 
rangement recall  somewhat  the 
glomeruli  of  the  kidney.  These 
areas  probably  represent  groups  ot 
imperfectly-developed  acini  ;  they 
are  well  seen  in  the  pancreas  of  man 
and  most  mammals. 

The  blood-vessels  of  the  pan- 
creas are  distributed  very  similarly 
to  those  of  the  salivary  glands.  The  larger  arterial  branches  run 
within  the  interlobular  connective  tissue,  sending  off  vessels  which 
pass  between  the  lobules  and  supply  the  glandular  parenchyma  with 
twigs.  These  latter  enter  the  lobules  and  form  net-works  which  en- 
close the  individual  acini  within  the  capillary  reticulum.  The  capil- 
laries lie  beneath  the  basement-membrane  in  close  relation  with  the 
glandular  epithelium.  The  veins  accompany  the  arterial  trunks  within 
the  connective  tissue. 

The  lymphatic  vessels  also  accompany  the  arteries,  lying  be- 
tween the  lobules  and  receiving  as  tributaries  the  lymph-radicles 
originating  within  the  lobule  between  the  acini.  The  larger  nerve - 
trunks  are  confined  to  the  connective  tissue  between  the  divisions 
of  the  gland,  in  which  situation  many  accompanying  microscopic 
ganglia  also  are  found.     The  ultimate  termination  of  the  nerve-fibres, 


Section  of  human  pancreas,  exhibiting  one 
of  the  areas  (a)  of  immature  gland-cells ;  b, 
the  usual  acini. 


THE   DIGESTIVE   TRACT.  Y$j 

as  in  the  case  of  the  salivary  glands,  is  still  undetermined ;  the 
fibrillae  are  traceable  to  the  basement-membrane  of  the  acini,  but 
their  further  accurate  disposition  remains  undecided. 

The  development  of  the  digestive  tract  and  its  appendages  in- 
volves all  three  blastodermic  layers,  the  mesoderm  and  the  ento- 
derm, however,  being  the  ones  participating  to  the  greatest  extent. 
The  epithelium  of  the  mucous  membrane,  together  with  that  of  the 
glandular  structures  connected  therewith,  is  the  direct  derivative  of 
the  entoderm,  with  the  exception  of  that  lining  the  oral  cavity  ante- 
rior to  the  fauces  and  the  salivary  and  oral  glands,  the  epithelium  of 
which  parts  originates  from  the  ectodermic  invagination.  For  a  short 
distance  within  the  anus,  likewise,  the  ectoderm  contributes  the  cells 
lining  the  gut.  As  already  pointed  out,  the  enamel  and  the  dentine 
are  also  products  respectively  of  the  ectoderm  and  of  the  mesoderm. 

The  formation  of  the  gut-tract  consists  essentially  of  a  process 
of   folding    off   and    closing 

together  of  the  ventral  body-  Fig.  232. 

plates,  which  are  composed 
of  the  entoderm  united  with 
the  visceral  layer  of  the  meso- 
derm. The  tube  thus  formed 
begins  in  the  cephalic  region 
of  the  embryo  as  a  blind, 
somewhat  dilated  pouch,  the 
primitive  pharynx,  which 
for  a  short  time  is  separated 
from  the  primary  oral  recess, 
or  stomodaeum,  by  a  parti- 
tion, the  pharyngeal  plate, 
consisting    of    the    opposed 

ectoderm  and  entoderm  ;  after  the  rupture  of  this  plate  the  gut-tract 
communicates  directly  with  the  exterior  through  the  oral  cavity.  A 
somewhat  similar  process  takes  place  at  the  lower  part  of  the  primitive 
digestive  tube,  whereby  the  anus  becomes  established.  For  a  con- 
siderable time  the  gut  communicates  with  the  cavity  of  the  umbilical 
vesicle  through  its  duct.  The  several  divisions  of  the  primary  diges- 
tive tube,  its  wall  consisting  of  epithelial  lining  and  supplementary 
mesodermic  tissue,  undergo  differentiation  and  acquire  distinctive 
characters,  which,  however,  depend  largely  upon  the  differentiation 
of  the  embryonal  epithelial  layer. 

The  division  of  the  tube  into  particular  regions  begins  with  the 
stomach,  which  as  early  as  the  fourth  week  in  the  human  embryo 
is  distinguishable  as  a  spindle-shaped  enlargement.  With  the  sub- 
sequent rapid  increase  in  the  size  of  the  organ,  the  tissues  constituting 


Transverse  section  of  nine-day  rabbit  embryo,  show- 
ing formation  of  primitive  gut  (g)  by  approximation  of 
ventral  plates  composed  of  visceral  layers  of  mesoderm 
and  entoderm  (e)  ;  m,  m,  body-cavity  bounded  by 
parietal  and  visceral  sheets  of  mesoderm  ;  n,  neural 
canal. 


NORMAL    HISTOLOGY. 


Fig.  233. 


its  walls  also  become  augmented  by  many  new  elements,  the  meso- 
dermic  cells  differentiating  into  a  narrow  looser  zone  next  the  ento- 
derm, which  later  becomes  the  submu- 
cosa,  and  a  broader,  more  compact 
stratum,  representing  the  future  mus- 
cular tunic.  The  entodermic  cells,  at 
first  arranged  as  a  single  layer,  soon 
undergo  local  proliferation,  the  resulting 
groups  of  cells  disposing  themselves  as 
minute  cylindrical  masses,  which  are 
the  earliest  traces  of  the  peptic  glands. 
These  increase  in  length  and  later  en- 
croach upon  the  underlying  mesoderm. 
In  the  young  gland  six  to  eight  tubular 
divisions  communicate  with  a  single 
duct,  but  as  development  advances  the 
ducts  divide,  with  a  corresponding  dimi- 
nution in  the  number  of  terminal  com- 
partments connected  with  each.  The 
pyloric  glands  appear  about  the  same 
time  as  do  the  peptic,  or  at  about  the 
tenth  week  of  fcetal  life,  the  cells  ac- 
quiring their  characteristic  form  and 
appearance  during  the  later  stages.  At 
first  and  during  a  considerable  period 
the  cells  lining  the  peptic  glands  are 
all  of  the  same  variety  ;  later  certain 
elements  become  distinguished  by  the 
accumulation  of  coarse  granules  within 
their  protoplasm  ;  these  constitute  the  acid  or  parietal  cells,  usually 
appearing  towards  the  close  of  the  fourth  month  of  fcetal  life. 

The  intestinal  divisions  of  the  primitive  gut  also  depend  for 
their  distinctive  characters  on  the  differentiation  of  the  entodermic 
epithelium  and  of  the  adjoining  mesoderm,  which  together  constitute 
the  mucosa.  The  villi,  distinguishable  by  the  tenth  week,  are  at 
first  relatively  short  and  thick  and  less  numerous  than  later,  when 
additional  projections  are  developed.  It  is  of  interest  to  note  that 
in  the  early  stages  villi  appear  in  both  the  large  and  the  small  intes- 
tine, these  structures  subsequently  atrophying  and  disappearing  in 
the  large  gut  while  they  increase  in  size  and  importance  in  the  re- 
maining parts  of  the  tube.  Coincidently  with  the  formation  of  the 
villi  the  entodermic  epithelium  sends  outgrowths  into  the  mesoderm 
between  the  villous  projections  ;  these,  at  first  solid,  cylinders  repre- 
sent the  early  stages  of  the  simple  tubular  glands  ;  with  the  gen- 


Sagittal  section  of  nine-day  rabbit 
embryo  :  B,  £',  neural  canal  and  brain 
vesicles;  »i,  ectodermic  invagination 
which  contributes  the  lining  of  anterior 
part  of  future  oral  cavity  ;  /,  primitive 
pharynx,  the  blind  upper  end  of  the 
primitive  gut  (g)  lined  with  entoderm, 
in  this  stage  separated  from  ectoderm 
by  septum  ;  U,  umbilical  duct  connect- 
ing gut  with  umbilical  vesicle  ;  //,  h' , 
arterial  and  venous  segments  of  young 
heart ;  delicate  endothelial  tube  seen 
lying  within  primitive  muscular  walls. 


THE    DIGESTIVE   TRACT. 


189 


eral  increase  in  the  thickness  of  the  young  mucosa  these  structures 
lengthen  and  obtain  their  lumen.  The  lower  ends  of  the  glands 
throughout  the  period  of  their  growth  are  the  seats  of  active  cell 
proliferation  and  the  points  at  which  the  division  of  their  fundi  com- 
mences in  the  production  of  the  compound  tubules.  The  endothe- 
lium covering  the  serous  surfaces  of  the  intestinal  tract  is  the  direct 
descendant  of  the  differentiated  mesoderm,  the  mesothelium,  lining 
the  body-cavity. 

The  development  of  the  accessory  glands  of  the  digestive 
tube,  including  the  liver,  the  pancreas,  and  the  salivary  glands, 
follows  the  same  general  plan.  The  epithelial  covering  of  the 
primitive  mucous  membrane  sends  cylindrical  masses  of  entodermic 
or  ectodermic  elements,  as  the  case  may  be,  into  the  surrounding 
mesoderm  ;  the  originally  single  cord  of  cells  very  soon  undergoes 
division,  a  richly-branched  system  of  epithelial  tubes  early  represent- 
ing the  future  gland.  The  liver  originates  as  a  ventral  outgrowth 
of  the  intestinal  epithelium  into  the  septum  transversum  ;  very  soon 
this  branches,  the  two  hepatic  diverticula  following  so  closely 
upon  the  stage  of  the  single  outgrowth  that  the  latter  is  sometimes 
overlooked.  The  walls  of  the  distal  ends  of  the  diverticula  soon 
become  greatly  thickened,  which  areas  of  entodermic  epithelium 
represent  the  earliest  traces  of  the  hepatic  tissue.  Regarding  the 
details  of  the  further  stages  in  the  growth  of  the  more  complicated 
livers  opinions  do  not  agree ;  it  is  probable,  however,  that  the 
hepatic  cords  of  the  mammalian  organ  are  attributable  to  the  same 
general  plan  of  development  as  are  other  tubular  glands,  the  com- 
plicated arrangement  of  the  secreting  tissue  resulting  from  incomplete 
separation  and  subsequent  fusion  of  the  cell-cords.  The  invasion  of 
the  epithelial  areas  by  the  blood-vessels  breaks  up  the  entodermic 
tissue  into  the  cell-nests  which  occupy  the  intercapillary  spaces. 
Two  forms  of  liver-cells  are  present  during  the  greater  part  of 
foetal  life,  large  polyhedral  elements,  and  small  round  cells,  the  latter 
disappearing  shortly  after  birth  ;  the  relation  between  the  two  varieties 
is  not  clearly  established,  but  the  small  cells  are  probably  younger 
stages  of  the  larger.  Multinucleated  cells  of  considerable  size 
also  occur  within  the  blood-vessels  of  the  embryonal  liver ;  these  are 
regarded  as  connected  with  the  production  of  red  blood-corpuscles 
before  birth.  The  lining  of  the  bile-vessels  and  of  the  interlobular 
bile-ducts,  together  with  the  hepatic  cells,  is  a  derivative  from  the 
entoderm,  while  the  connective  tissue  and  blood-vessels,  as  well  as 
the  tissues  of  the  walls  of  the  bile-vessels  other  than  the  epithelial 
lining,  are  contributions  from  the  mesoderm. 

The  pancreas  appears  shortly  after  the  liver  as  a  dorsal  diver- 
ticulum, which  extends  from  the  gut  into  the  primitive  omentum, 


190 


NORMAL    HISTOLOGY. 


or  mesogastrium,  sending  out  hollow  buds  and  lateral  branches. 
The  organ  first  lies  parallel  to  the  sagittal  axis  of  the  body,  afterwards 
changing  its  position  so  as  to  lie  transversely,  the  former  anterior  ex- 
tremity passing  to  the  left.  In  many  mammals  ventral  diverticula 
appear  in  addition  to  the  dorsal  outgrowth  :  to  what  extent  these  are 
formed  in  man,  and  to  which  portions  of  the  organ  they  contribute,  is 
still  uncertain.  The  presence  of  more  than  one  pancreatic  duct  in  cer- 
tain animals  is  explained  by  the  persistence  of  the  embryonal  condition. 
The  tubular  acini  of  the  organ  are  developed  in  a  manner  similar  to 
that  in  which  those  of  the  other  salivary  glands  are  formed  :  the 
cylinders  of  entodermic  cells  send  off  branches,  which,  in  turn,  give 
rise  to  secondary  buds,  the  lumen  of  the  original  diverticulum  ex- 
tending into  the  terminal  compartments  of  the  gland.  The  ingrowth 
of  the  surrounding  mesoderm  establishes  the  division  into  lobules 
and  supplies  the  interlobular  connective  tissue. 


THE  URINARY  ORGANS. 


I9I 


CHAPTER    XI. 


THE   URINARY    ORGANS. 


THE   KIDNEY. 


The  kidney  is  a  highly-developed  compound  tubular  gland,  com- 
posed of  pyramidal  lobules  which  correspond  in  number  with  the 
renal  papillae  and  Malpighian  pyramids  :  in  the  adult,  however,  their 

distinctness  is  lost,  since 

Fig.  234. 

h  l,™ 


they  become  blended  to- 
gether. On  laying  open 
the  fresh  organ  by  a 
longitudinal  section, 
two  regions  are  ap- 
preciable, the  cortex 
and  the  medulla.  The 
cortex  is  readily  distin- 
guished as  the  periph- 
eral granular  zone  em- 
bracing the  outer  third, 
while  the  medulla  ap- 
pears radially  striated 
and  occupies  the  re- 
maining two-thirds  of 
the  gland. 

The  inner  surface  of 
the  medulla,  next  the 
pelvis,  presents  a  num- 
ber of  eminences,  or 
papillae,  at  whose  apices 
open  the  large  terminal 
uriniferous  tubules  or 
excretory  ducts.  Each 
renal  papilla  is  the  cul- 
minating point  of  a  sys- 
tem of  dividing  and  sub- 
dividing tubules,  which 
collectively  form  a  pyramidal  mass,  the  base  of  which  corresponds 
to  the  surface  of  the  organ,  while  its  apex  is  the  papilla.  These 
pyramidal  tracts  constitute  the  lobules  of  which  the  kidney  is  com- 


Longitudinal  section  of  human  kidney,  exhibiting  general 
relations  of  macroscopic  details  :  A,  renal  artery  ;  U,  ureter  ; 
C,  one  of  the  calices  into  which  a  papilla  projects ;  1,  cortex 
containing  labyrinth  (/)  and  medullary  rays  (m)  ;  2,  medulla  ; 
M,  Malpighian  pyramids,  some  obliquely  cut  at  3  ;  b,  bound- 
ary layer;  B,  columns  of  Bertini  ;  4,  masses  of  adipose  tissue; 
5,  branches  of  renal  artery.     (After  Henle.) 


192 


NORMAL   HISTOLOGY. 


posed.  In  the  adult  human  organ  all  traces  of  such  divisions  have 
usually  disappeared ;  during  foetal  life,  however,  the  lobules  are  dis- 
tinctly seen,  a  condition  which  is  permanently  retained  in  many  of 
the  lower  animals. 

The  medulla  is  occupied  by  8-18  striated  conical  Malpighian 
pyramids,  the  apices  of  which  correspond  to  the  papillae,  while  their 
bases  occupy  the  line  of  juncture  between  the  cortex  and  the  medulla. 

Each     pyramid    exhibits 
Fig.  235.  alternating  light  and  dark 

striae,  these  markings 
being  respectively  the 
uriniferous  tubules  and 
the  blood-vessels.  The 
masses  of  the  organ  ex- 
tending between  the  sides 
of  the  Malpighian  pyra- 
mids as  far  as  the  pelvis 
constitute  the  columns 
of  Bertini,  and  are  trav- 
ersed by  the  large  blood- 
vessels. 

At  certain  points  along 
their  bases  the  striae  of  the 
Malpighian  pyramids  are 
continued  into  the  cortex 
as  slender,  tapering  bun- 
dles of  parallel  tubules, 
which  form  the  medul- 
lary rays,  or  pyramids 
of  Ferrein.  By  the 
penetration  of  these  bun- 
dles the  cortex  is  sub- 
divided into  the  med- 
ullary rays  and  the 
labyrinth,  the  latter  ap- 
propriately so  named  on 
account  of  the  great  tortuosity  of  the  component  uriniferous  tubules. 
The  dark-red  points  irregularly  studded  over  the  labyrinth  indicate 
the  position  of  the  Malpighian  bodies.  In  sections  parallel  to 
the  free  surface  the  medullary  rays  appear  as  groups  of  tubules  sur- 
rounded by  the  labyrinth  on  all  sides. 

The  blood-vessels  of  the  labyrinth  are  enveloped  in  connective 
tissue,  which  latter  represents  the  interlobular  tissue  of  other 
glands  and  the  boundaries  of  the  primary  lobules.     The  secreting 


vJJg}')lijii\\iA'\il,4  V/\\\'! ii*)ii  li'ii'it/ii/S, i 'I'iUiiiilWMYlX 

M 
Section  of  human  kidney,  including  cortex  and  portion  of 
medulla,  showing  general  arrangement  of  tissues.  Cortex 
(C)  is  imperfectly  subdivided  by  bundles  of  parallel  tubules 
constituting  the  medullary  rays  (**)  ;  between  these  lies  the 
labyrinth  (/)  containing  the  Malpighian  bodies  (x)  ;  in  places 
(x')  the  glomerulus  has  fallen  out,  leaving  the  empty  capsule  ; 
b  and  v,  sections  of  blood-vessels. 


THE  URINARY   ORGANS.  tqi 

parenchyma  of  the  organ  is  held  in  place  by  the  interstitial  con- 
nective tissue  ;  this  is  present  between  the  tubules  in  most  parts  of 
the  kidney  in  very  small  quantities, — the  immediate  vicinity  of  the 
Malpighian  bodies  and  the  papillary  region  of  the  medulla  being 
exceptions,  since  considerable  amounts  of  the  interstitial  tissue  are 
present  in  these  localities.  The  connective  tissue  of  the  kidney  be- 
comes condensed  at  the  periphery  of  the  organ,  where  it  forms  a 
fibrous  investment,  over  which,  in  addition,  the  special  capsule 
extends. 

The  Malpighian  bodies  are  situated  exclusively  within  the  cortex, 
and  are  limited  to  the  labyrinth.  They  consist  of  two  parts  — a 
spherical  mass  of  convo- 
luted capillary  blood-ves-  FlG-  236- 
sels,  the  glomerulus,  or 
the  Malpighian  tuft,  and 
the  surrounding  expanded 
extremity  of  the  uriniferous 
tubule,  the  capsule  of 
Bowman.  The  glomer- 
ulus is  supplied  by  an 
afferent  artery,  which 
divides  into  several 
branches ;  each  of  these 
breaks  up  into  numerous 
capillaries,  which  are 
united  by  delicate  con- 
nective tissue  into  groups 
or  lobules.  The  blood 
escapes  from  the  convo- 
luted capillaries  of  the 
glomerulus  by  the  effer- 
ent vessel,  which  passes 
out  by  the  side  of  the  en- 
tering artery. 

The  glomerulus,  as 
usually  seen  in  sections, 
seems  to  lie  within  the 
capsule,  the  blood-vessels  having  apparently  pierced  the  latter  to 
gain  entrance.  The  vessels,  however,  really  are  outside  the  cavity 
of  the  capsule,  since  one  surface  of  this  structure  has  been  pushed  in 
before  the  advancing  tuft  during  its  development.  The  masses  of 
convoluted  capillaries  are  closely  invested  by  the  reflected  portion 
of  the  capsule,  which  likewise  dips  in  between  the  vascular  lobules 
of  the  glomerulus.     The  invaginated  portion  becomes  continuous 


Section  of  human  kidney  partially  injected  :  a,  interlobu- 
lar artery  giving  off  afferent  twig  (b) ;  c,  efferent  vessel 
passing  into  intertubular  capillaries  (d) ;  e,  convoluted 
capillaries  of  glomerulus  ;  _/",  outer  layer  of  Bowman's  cap- 
sule, the  nuclei  of  whose  cells  show  at  g;  h,  uriniferous 
tubule  in  transverse  section,  i,  in  oblique  section. 


194 


NORMAL   HISTOLOGY. 
Fig.  237. 


Pelvis 

Diagram  of  the  kidney,  showing  the  course  of  the  uriniferous  tubules  and  of  the  blood  vessels ;  for 
convenience  the  medulla  is  represented  as  greatly  shortened.  The  various  divisions  of  the  tubule- 
Bowman's  capsule,  neck,  proximal  convoluted,  spiral,  descending  and  ascending  limbs  and  loop  of 
Henle's  loop,  irregular,  distal  convoluted,  arched  collecting,  straight  collecting,  and  excretory  duct- 
arc  indicated  by  their  initial  letters  :  a,  e,  and  c,  respectively  the  afferent,  efferent,  and  capillary 
blood- vessels  ;  s,  stellate  vein  ;  v  r,  vasa  rectae. 


THE  URINARY   ORGANS.  IO/r 

with  the  outer  layer  of  the  capsule  at  the  stalk  of  the  glomerulus,  at 
which  point  the  vessels  and  the  capsule  are  intimately  united. 

Each  uriniferous  tubule  begins  within  the  labyrinth  as  the 
dilated  capsule  of  Bowman.  A  greatly  constricted  neck,  situated 
at  the  pole  of  the  Malpighian  body  opposite  the  position  of  the 
vascular  stalk,  leads  into  the  first  or  proximal  convoluted  tubule, 
which  is  characterized  by  its  considerable  size  and  tortuous  course. 
Leaving  the  labyrinth,  to  which  it  has  thus  far  been  confined,  the 
tubule  enters  the  medullary  ray  and  passes  towards  the  medulla  as 
the  slightly  wavy  spiral  portion  ;  on  reaching  the  medulla  a  marked 
diminution  in  the  size  of  the  tubule  takes  place,  the  reduced  tube 
passing  into  the  medulla  as  far  as  the  papillary  zone  as  the  descend- 
ing limb  of  Henle's  loop,  the  narrowest  part  of  the  entire  urinifer- 
ous tubule.  The  spiral  tubule  is  practically  the  beginning  of  the 
descending  limb  of  Henle's  loop,  and  takes  the  place  of  this  arm  in 
the  medullary  ray,  into  the  constitution  of  which,  strictly  regarded, 
it  does  not  enter. 

Just  before  reaching  the  loop  itself  the  tubule  becomes  slightly 
larger,  obtaining  a  diameter  which  is  retained  throughout  the  loop 
and  the  ascending  limb  ;  on  again  reaching  the  cortex,  the  ascend- 
ing limb  enters  the  medullary  ray  as  its  second  constituent  until  it 
once  more  enters  the  labyrinth,  to  become,  for  a  short  distance,  the 
conspicuous  irregular  tubule.  The  succeeding  second  or  distal 
convoluted  portion  resembles  very  closely  the  proximal  part  of 
like  name,  possessing  a  similar  size  and  tortuous  course.  The  uri- 
niferous tubule  finally  leaves  the  labyrinth  as  the  arched  collecting 
tubule,  to  enter,  for  the  third  time,  the  medullary  ray  as  the  straight 
collecting  tube.  In  consequence  of  the  frequent  union  of  canals  of 
smaller  size,  the  collecting  tubes  rapidly  increase  in  diameter  as  they 
traverse  the  medulla,  until,  in  the  papillary  layer,  the  narrow  tubules 
have  become  the  large  excretory  ducts,  or  tubes  of  Bellini,  whose 
orifices  on  the  free  surface  of  the  papillae  are  recognizable  by  the 
unaided  eye.  A  certain  number  of  tubules  probably  do  not  form 
loops  of  Henle,  but  pass  directly  to  become  the  collecting  canals 
(Rose). 

From  the  foregoing  it  will  be  seen  that  the 

Malpighian  bodies — glomeruli  and  cap- 
sules ; 
Constricted  ?tecks  of  tubules  ; 
Proximal  convoluted  tubules  ; 
Irregular  tubules  ; 
Distal  co7ivoluted  tubules  ; 
^  Arched  collecting  tubules. 


Labyrinth  contains 


196 

Medullary  ray  contains  : 

Medulla  contains  : 


NORMAL    HISTOLOGY. 
(  Spiral  tubules  ; 


Ascendi?ig  limbs  of  Henle'  s  loops  ; 

Straight  collecting  tubules. 
c  Descending  limbs  of  Henle'  s  loops  ; 
!    The  loops  ; 

Ascending  limbs  of  the  loops  ; 

Collecting  tubules  of  all  sizes. 


While  the  labyrinth  is  characterized  by  the  irregular  and  tortuous 
course  of  its  tubules,  the  medullary  ray  and  the  medulla  are  dis- 
tinguished by  the  longitudinal,  generally  parallel  arrangement  of  their 
components. 

The  wall  of  all  parts  of  the  tubule  consists  of  the  basement-mem- 
brane and  the  lining  epithelium  ;  the  variations  in  the  character  of 
the  latter  are  so  numerous  that  it  is  desirable  to  consider  each  portion 
of  the  tubule  in  detail. 

1.  The  capsule,  the  expanded  and  invaginated  blind  termination 
of  the  uriniferous  tubule,  is  lined  with  a  single  layer  of  large,  flattened 
epithelium,   resembling  endothelial  plates.      This  covers,   likewise, 

Fig.  238. 


Portions  of  the  various  divisions  of  the  uriniferous  tubules  drawn  from  sections  of  human  kidney; 
A,  Malpighian  body  ;  x,  squamous  epithelium  lining  the  capsule  and  reflected  over  the  glomerulus  ; 
y,  z,  afferent  and  efferent  vessels  of  the  tuft ;  e,  nuclei  of  capillaries ;  n,  constricted  neck  marking 
passage  of  capsule  into  convoluted  tubule  ;  B,  proximal  convoluted  tubule ;  C,  irregular  tubule  ;  D 
and  F,  spiral  tubules  ;   E,  ascending  limb  of  Henle's  loop  ;  G,  straight  collecting  tubule. 

the  portion  reflected  over  the  glomerulus.  In  ordinary  preparations 
the  presence  of  the  cells  is  indicated  by  the  delicate  spindle  nuclei 
seen  in  profile ;  the  numerous  nuclei  seen  within  the  tissues  of  the 


THE  URINARY   ORGANS.  jg* 

glomerulus  include  those  of  the  walls  of  the  blood-vessels  and  of  the 
interstitial  tissue,  as  well  as  those  of  the  capsular  epithelium. 

2.  At  the  neck  the  flattened  epithelium  abruptly  becomes  cuboidal 
and  rapidly  assumes  the  character  of  the  lining  of  the  convoluted 
tubule.  The  existence  of  ciliated  epithelium  at  the  neck  or  within 
the  capsule  in  the  mammalian  kidney  has  been  asserted,  but  not  satis- 
factorily established  ;  in  many  of  the  lower  animals,  however,  as  in 
the  amphibians,  the  presence  of  cilia  is  readily  demonstrated,  as  is  like- 
wise the  existence  of  tubules  opening  directly  into  the  peritoneal  cavity. 
Such  trumpet-shaped  orifices — the  nephrostomata — represent  a 
partial  persistence  of  the  primitive  type  of  excretory  organ,  in  which 
the  tubules  pass  directly  from  the  body-cavity  to  the  outer  surface. 

3.  The  proximal  convoluted  tubule  is  clothed  with  low  co- 
lumnar or  cuboidal  cells,  whose  granularity  and  transparency  vary 
with  the  stage  of  secretion,  as  do  likewise  the  thickness  of  the  epi- 
thelium and  the  size  of  the  lumen  of  the  canal.     The  outer  zone  of 


Section  of  kidney  of  amphiuma :  the  peritoneal 
surface  (b,  6)  exhibits  one  of  the  nephrostomata 
(<?),  lined  with  ciliated  cells ;  d,  glomerulus  sur- 
rounded by  capsule  ;  ti,  uriniferous  tubules  ;  v, 
capillaries  filled  with  red  blood-cells. 


Portions  of  the  constituents  of  the  medulla 
from  the  human  kidney  :  A ,  B,  collecting 
tubules  ;  C,  D,  descending  and  ascending 
limbs  of  Henle's  loop  ;  E,  blood-vessel. 


the  epithelium,  next  the  basement-membrane,  presents  more  or  less 
clearly  the  vertical  striation  distinguishing  rod-epithelium.  The 
demarcation  of  the  individual  cells  is  not  sharply  marked,  their 
boundaries  being  indistinctly  defined. 

4.  The  epithelium  of  the  spiral  tubule  closely  resembles  that  of 
the  preceding  portion,  consisting  of  similar  low  columnar  elements 
possessing  granular  protoplasm  but  less  marked  striations. 


jg3  NORMAL   HISTOLOGY. 

5.  The  conspicuous  diminution  in  diameter  which  marks  the  pas* 
sage  of  the  spiral  tubule  into  the  descending  limb  of  Henle's  loop 
is  accompanied  by 'a  change  in  the  character  of  the  lining  epithelium. 
The  low  columnar  cells  are  replaced  by  flattened,  transparent  plates, 
whose  nuclei,  thicker  than  the  bodies  of  the  cells,  encroach  upon  the 
lumen  of  the  tubule  as  minute  spindle-shaped  projections  ;  since  the 
latter  are  situated  often  on  opposite  sides  of  the  tube,  its  lumen  in 
section  appears  as  a  wavy  channel. 

6.  Shortly  before  reaching  the  loop,  at  a  point  within  the  de- 
scending limb  corresponding  with  the  increased  diameter  of  the 
tubule,    the   epithelium   becomes   polyhedral,    possessing    flattened 


nuclei   and  faint   striations ;    the 


Fig.  241. 


Section  of  medulla  of  human  kidney  :  w,  large  collect- 
ing tubules  ;  x  and_y,  descending  and  ascending  limbs  of 
Henle's  loops  ;  z,  loops  of  Henle  ;  v,  blood-vessels. 


lumen  is  distinct  in  this  region, 
although  narrow.  This 
character  is  retained  by  the 
epithelium  throughout  the 
loop  and  the  ascending 
limb  as  far  as  the .  succeed- 
ing portion  of  the  tube. 

7.  The  irregular  tubule 
is  distinguished  by  its  small 
and  uncertain  lumen  and 
its  distinctly  striated  epi- 
thelium ;  the  thickness  of 
the  latter  and,  conse- 
quently, the  size  of  the 
canal  vary  with  the  con- 
ditions of  secretion. 

8.  The  lining  of  the  dis- 
tal convoluted  tubule 
resembles  that  of  the  proxi- 
mal, the  epithelium  being 
granular,  indistinctly  sepa- 
rated into  individual  cells, 
and  presenting  a  striated 
outer  zone  ;    the  lumen  of 


the  canal  depends  largely 
upon  the  thickness  of  the  lining  cells,  which  changes  with  the  func- 
tional activity  of  the  secretory  elements. 

9.  The  succeeding  segment,  the  arched  collecting  tubule,  con- 
tains low  cuboidal,  transparent  cells,  which,  with  slight  alteration, 
become  the  epithelium  of  the  straight  collecting  tubule. 

10.  Passing  into  the  medulla,  the  cells  of  the  collecting  tubules 
become  markedly  columnar,  which  form  they  retain  with  increasing 
distinctness  throughout  the  remainder  of  their  course.      The  large 


THE   URINARY   ORGANS. 


199 


excretory  ducts,  or   tubes   of  Bellini,   in   the   papillary  region 
present  a  beautiful  example  of  simple  columnar  epithelium  in  the 
tall,  transparent,  and  clearly-defined  cells  with  which  they  are  lined. 
These  cells,  the  largest  epi- 
thelial elements  within  the  Fig.  242. 
kidney,    are    defined    from 
one  another  with  great  dis- 
tinctness, and  possess  oval 
nuclei     situated    somewhat 
their  outer  bound- 


nearer 
aries. 

The  blood-vessels  with- 
in the  kidney  are  very  plen- 
tiful. The  renal  artery, 
entering  at  the  hilum,  passes 
through  the  sinus  within 
the  submucous  tissue  which 
occupies  the  space  between 
the  wall  of  the  pelvis  and 
the  neighboring  paren- 
chyma ;  during  its  course 
through  the  sinus  several 
small  twigs  are  given  off  for 


Transverse  section  of  papillary  region  of  medulla  of  hu- 
man kidney,  more  highly  magnified  :  C,  large  collecting 
tubules ;  x  and  y,  descending  and  ascending  limbs  of 
Henle's  loops  ;  v,  blood-vessels. 


the  nutrition  of  the  struc- 
tures in  the  immediate  vicinity.  Before  entering  the  glandular  tissue 
the  renal  artery  breaks  up  into  a  number  of  large  branches,  which 
traverse  the  parenchyma  through  oblique  channels  within  the  inter- 
pyramidal  tracts,  or  columns  of  Bertini,  to  gain  a  position  at  the 
juncture  of  the  cortex  and  medulla  corresponding  to  the  bases 
of  the  Malpighian  pyramids.  At  this  point  they  bend  sharply  to 
form  a  series  of  horizontal  arches,  from  which  two  sets  of  vessels 
spring— the  ascending  interlobular  cortical  arteries  and  the 
arteriae  rectae  of  the  medulla. 

The  straight  cortical  branches,  passing  towards  the  free  surface 
of  the  organ,  give  off  short,  curved  lateral  twigs  to  supply  the  affer- 
ent vessels  of  the  glomeruli.  These  branches  divide  into  groups 
or  lobules  of  convoluted  capillaries  ;  the  latter,  in  turn,  join  to  form 
the  slightly  smaller  efferent  vessels,  which  carry  off  the  still  arterial 
blood  from  the  Malpighian  bodies.  The  efferent  vessels  soon  break 
up  into  capillary  net-works  which  surround  the  tubules  of  the 
labyrinth  and  the  medullary  ray.  These  net-works  are  taken  up  by 
the  interlobular  veins  which  accompany  the  arteries,  and  pass  to 
the  pelvis,  where  they  aid  in  forming  the  large  renal  veins.  The 
vessels  collecting  the  blood  from  the  peripheral  zone  of  the  cortex 


200 


NORMAL   HISTOLOGY. 


converge  to  certain  points,  where  they  form  the  venae   stellatae  ; 
these  veins  afterwards  pass  into  the  labyrinth  and  follow  the  inter- 
lobular vessels. 

The  arteries  supply- 
ing the  medulla  enter 
as  straight  vessels,  the 
arteriae  rectae,  which 
undergo  repeated  divis- 
ion to  form  rich  inter- 
lobular net-works  reach- 
ing as  far  as  the  papillae, 
where  the  orifices  of  the 
excretory  ducts  are  sur- 
rounded by  capillaries. 
The  blood  within  the 
medulla  is  collected  by 
the  venae  rectae,  which 
accompany  the  corre- 
sponding arteries  and 
empty  into  the  large 
veins  situated  at  the 
juncture  of  the  cortex 
and  the  medulla.  The 
large  venous  trunks 
pass  obliquely  through 
the  medulla,  along  with 
the  arteries,  to  reach  the 
pelvis,  where  they  join 
with  their  fellows  to  form 
the  renal  veins. 

The  lymphatics  of 
the  kidney  are  arranged  as  two  sets  of  vessels ;  a  superficial  sys- 
tem ramifies  within  the  deeper  layers  of  the  capsule,  while  a  system 
of  deeper  channels  passes  in  company  with  the  blood-vessels  into 
the  interior  of  the  organ  to  communicate  with  the  numerous  lym- 
phatic clefts  and  spaces  which  exist  within  the  intertubular  connective 
tissue. 

Regarding  the  ultimate  distribution  of  the  nerves  of  the  kidney, 
little  is  known  with  certainty  beyor^  the  fact  that  they  enter  the 
parenchyma  in  company  with  the  blood-vessels,  around  which  they 
form  net-works  of  non-medullated  fibres;  the  nerve-fibres  have  been 
traced  between  the  tubules,  where  they  form  meshes  immediately 
outside  the  membrana  propria.  The  ultimate  distribution  of  the 
fibrillar  and  their  relations  to  the  secreting  cells  are  still  uncertain. 


Section  of  injected  kidney  of  dog,  showing  general  disposi- 
tion of  blood-vessels :  a  and  i,  large  arterial  and  venous 
branches  situated  at  junction  of  cortex  (C)  and  medulla  ( M), 
which  break  up  into  ascending  interlobular  twigs  (c)  and  de- 
scending straight  vessels  (/,  /')  ;  e,f,  afferent  and  efferent  ves- 
sels of  glomeruli  (g)  ;  h,  intertubular  capillary  net-works ;  i, 
peripheral  venous  trunks,  which  collect  the  blood  from  sub- 
capsular net-works  (k). 


THE   URINARY   ORGANS. 


201 


THE   RENAL   SINUS   AND    THE   URETER. 

The  greater  part  of  the  renal  sinus  is  occupied  by  the  dilated, 
pouch-like  expansion  of  the  upper  extremity  of  the  excretory  duct 
of  the  kidney,  the  ureter,  embracing  the  pelvis  and  its  subdivisions, 
the  calices,  and  the  infundibula.  These  cavities,  together  with  the 
protruding  portions  of  the  renal  papillae,  are  invested  by  a  membra- 
nous structure  consisting  of  three  coats,  the  mucous,  the  muscu- 
lar, and  the  fibrous.  The  mucous  coat  is  covered  with  stratified 
squamous  epithelium,  which  comprises  relatively  few  layers  of  cells, 
and  is  frequently  termed  "transitional,"  in  view  of  the  rapid  change 

Fig.  244. 


fZ^: -^SsS-^S^; 


■J/wm%   mam?  ;-^>^g-   ■  ■■  \  J  \ 


H 


Transverse  section  of  human  ureter :  a,  irregular  lumen,  lined  by  mucous  membrane,  which  con- 
sists of  epithelium  {b),  tunica  propria  (V),and  submucous  tissue  (d);  e,  f,  longitudinal  and  circular 
bundles  of  muscular  tunic ;  g,  additional  longitudinal  muscular  bundles  ;  h,  fibrous  tissue ;  i,  blood- 
vessels. 

from  the  columnar  elements  of  the  deep  layer  to  the  squamous  ones 
of  the  superficial  stratum.  The  tunica  propria,  or  stroma  of  the 
mucous  membrane,  consists  of  a  felt-work  of  fibro-elastic  bundles, 
contains  a  few  small  racemose  glands,  and  passes  insensibly  into 
the  inconspicuous  submucous  tissue.  The  muscular  coat  is 
arranged  as  an  inner  longitudinal  and  an  outer  circular  layer 
composed  of  bundles  of  involuntary  muscle  cells.  On  the  papillae 
the  circular  bundles  are  especially  well  developed,  enclosing  the  renal 
tissue  somewhat  as  a  sphincter.  The  outer  fibrous  coat  consists  of 
irregularly-placed  bundles  of  connective  tissue,  which  connect  the 
organ  with  the  surrounding  structures.      The  walls  of  the  ureter 


202 


NORMAL   HISTOLOGY. 


m 


proper  contain  the  same  layers  that  are  found  in  the  pelvic  portion 
of  the  tube ;  the  muscular  tunic,  however,  is  somewhat  better  de- 
veloped, and  in  the  lower  part  of  the  ureter  is  augmented  by  an  addi- 
tional imperfect  external  longitudinal  layer,  although  the  latter  is 
represented  in  places  by  only  a  few  scattered  bundles  of  non-striped 
muscle. 

THE   URINARY   BLADDER. 

The  bladder  is  composed  of  the  same  coats  that  are  found  in 
the  ureter,  the  mucous,  the  muscular,  and  the  fibrous,  together 
with  the  serous  surface  in  those  parts  of  the  organ  which  possess 
a  peritoneal  covering.     The  epithelium  of  the  bladder  corresponds 

to  that  lining   the  ureter  and 
Fig.  245.  the  renal  pelvis,  being  of  the 

stratified  squamous  ' '  transi- 
tional" type.  The  mucous 
membrane  at  the  base  of  the 
bladder  contains  small  race- 
mose glands ;  minute  lymph- 
follicles  are  also  found  within 
the  mucosa.  The  involun- 
tary muscle  is  arranged  in 
three  general  layers,  an  in- 
ner and  an  outer  longitu- 
dinal enclosing  a  middle 
circular  stratum ;  the  bun- 
dles composing  these  tunics 
are,  however,  so  irregularly 
disposed  that  the  layers  are 
very  imperfectly  defined.  At 
the  base  of  the  organ  the  inner 
longitudinal  muscular  bundles 
increase  in  size,  while  those  of 
the  augmented  circular  layer 
constitute  the  internal  vesical 
sphincter. 

The  blood-vessels  of  the 
ureter  and  the  bladder  sup- 
ply the  muscular  and  mucous 
coats  with  rich  capillary  net- 
works, the  one  situated  within 
the  mucosa  of  the  bladder  being  especially  rich.  The  lymphatics 
have  much  the  same  distribution  as  have  the  blood-vessels,  net- 
works being  found  within  the  deeper  layers  of  the  mucosa,  as  well  as 
more  sparingly  within  the  muscular  tissue.     The  nerves  supplying 


Section  of  human  bladder:  a,  squamous  epithelium 
covering  the  folds  of  the  tunica  propria  ;  b,  submucous 
tissue ;  c,  d,  irregularly-disposed  circular  and  longi- 
tudinal bundles  of  non-striped  muscle  ;  e,  fibrous  tis- 
sue of  serous  coat. 


THE  URINARY   ORGANS.  203 

the  ureter  and  the  bladder  are  largely  composed  of  sympathetic  fila- 
ments comprising  both  medullated  and  non-medullated  fibres.  The 
fibres  pass  into  the  mucosa  as  far  as  the  epithelium  ;  but  whether 
they  penetrate  between  the  epithelial  cells  is  still  undetermined. 
Numbers  of  microscopic  ganglia  are  situated  along  the  course  of 
the  nerves  of  the  bladder,  this  organ  affording  in  smaller  animals  a 
favorable  situation  for  studying  ganglion-cells  and  nerve-fibres,  as 
well  as  involuntary  muscle  cells. 

THE   URETHRA. 

The  urethra,  both  male  and  female,  consists  of  a  mucous  coat, 
strengthened  by  a  variable  muscular  tunic  and  by  fibrous  tissue. 

The  female  urethra  is  lined  throughout  by  a  stratified  squa- 
mous epithelium,  which  rests  upon  a  basement-membrane  cover- 
ing the  numerous  small  papillae  with  which  the  surface  of  the  tunica 
propria  is  beset.  These  papillae  are  especially  plentiful  and  well 
developed  near  the  termination  of  the  canal,  in  the  vicinity  of  the 
meatus.  The  tunica  propria,  or  stroma  of  the  mucosa,  is  composed 
of  interwoven  bundles  of  fibrous  and  elastic  fibres,  the  superficial 
layers  of  which,  particularly  in  the  vicinity  of  the  internal  orifice,  are 
infiltrated  with  lymphoid  cells.  Small  acinous  glands  are  sparingly 
present.  The  muscular  tunic  is  well  developed  and  arranged 
as  an  inner  longitudinal  and  an  outer  circular  layer  of  non- 
striped  muscle.  The  intermuscular  connective-tissue  lamellae  contain 
many  elastic  fibres. 

The  male  urethra  is  lined  with  epithelium,  the  character  of  which 
varies  in  the  different  portions  of  the  canal.  In  the  prostatic  part 
the  epithelium  resembles  that  lining  the  bladder,  being  of  the  transi- 
tional variety ;  this  passes  gradually  into  the  stratified  columnar 
type  of  the  investment  of  the  membranous  part,  which  in  turn 
gives  place  to  a  single  layer  of  simple  columnar  cells  in  the  penile 
portion.  The  fossa  navicularis  is  lined  with  stratified  squamous 
epithelium  continuous  with  that  covering  the  glans.  The  tunica 
propria  bears  numerous  papillae,  which  are  particularly  well  devel- 
oped within  the  navicular  fossa.  The  small  racemose  glands  of 
Littre  are  found  through  the  entire  urethra.  Inner  longitudinal 
and  outer  circular  bundles  of  non-striped  muscle  surround  the  pros- 
tatic portion,  extending  over  the  membranous  part  to  be  lost  on  the 
spongy.  In  addition,  the  fibres  of  the  compressor  urethrae  muscle 
contribute  a  distinct  muscular  investment  for  the'membranous  por- 
tion, which  fades  away  at  either  border  over  the  penile  and  prostatic 
segments.  The  anterior  part  of  the  penile  division  is  destitute  of 
muscular  tissue.  Outside  of  the  muscular  layer  a  variable  fibrous 
tunic  gives  additional  firmness  and  strength  to  the  canal.     The  walls 


204 


NORMAL    HISTOLOGY. 


of  the  urethra  are  liberally  supplied  with  blood-vessels,  which  form 
rich  capillary  net-works  beneath  the  epithelium.  The  larger  lym- 
phatics lie  in  the  submucosa,  where  they  receive  the  radicles  accom- 
panying the  blood-vessels  within  the  mucosa.  The  nerves  which 
bear  the  blood-vessels  company  find  their  endings  within  the  super- 
ficial sub-epithelial  layer  of  the  mucosa. 

The  Development  of  the  Urinary  Organs.  In  tracing  the 
history  of  the  formation  of  these  structures  the  genesis  of  three 
distinct  divisions  must  be  considered — the  development  of  the 
kidney  and  ureter,  that  of  the  bladder,  and  that  of  the  urethra. 

The  permanent  kidney  is  preceded  in  the  embryo  by  an  important 
although  transient  excretory  organ,  the  Wolffian  body  ;  the  prod- 
ucts of  this  organ  are  carried  off  and  emptied  into  the  primitive  intes- 
tinal canal  by  its  excretory  tube,  the  Wolffian  duct.  All  parts  of 
the  Wolffian  body  and  the  duct  consist  of  mesodermic  tissue  alone, 
these  structures  arising  essentially  as  outgrowths  from  the  primitive 
peritoneal  lining  into  the  surrounding  mesoderm.  The  mesothelial 
evaginations  so  originating  constitute  the  primary  tubules  of  the 
Wolffian  body,  from  which  numerous  secondary  canals  are  derived ; 
the  subsequent  development  of  blood-vessels  in  intimate  relation 
with  the  tubules  produces  the  primitive  Mal- 
pighian  corpuscles  of  the  foetal  organ. 

The  first  step  in  the  development  of  the 
kidney  consists  in  a  dorsal  outgrowth  from 
the  Wolffian  duct  near  its  cloacal  end  ;  this  diver- 
ticulum grows  forward  and  parallel  with  the 
Wolffian  duct  until  its  extremity  reaches  a 
position  behind  and  somewhat  above  the  caudal 
end  of  the  Wolffian  body.  The  primary  kidney- 
tube  now  expands  at  its  upper  end,  the  dilated 
portion  subsequently  undergoing  peripheral 
cleavage  into  a  number  of  tubular  compart- 
ments. Coincidently  with  the  growth  and  dif- 
ferentiation of  the  epithelial  evagination  from 
the  Wolffian  duct,  the  mesodermic  tissue  into 
which  the  expanded  extremity  of  the  diverticu- 
lum makes  its  way  becomes  greatly  condensed. 
The  fundamental  structures  in  the  development 
of  the  kidney  and  the  ureter  are  now  distinctly 
defined.  The  narrow,  elongated  portion  of  the 
outgrowth  from  the  Wolffian  duct  becomes  the 
epithelial  lining  of  the  ureter,  while  the  ex- 
panded terminal  part  forms  that  of  the  pelvis  and  of  the  urinifer- 
ous  tubules.     The  connective  and  vascular  tissues  are  derived  from 


Fig.  246. 


Sagittal  section  of  eleven- 
day  rabbit  embryo,  show- 
ing earliest  stage  of  de- 
velopment of  kidney  as 
outgrowth  (k)  from  Wolffian 
duct  (»)  into  surrounding 
mesoderm  (>«). 


THE  URINARY  ORGANS. 


205 


the  surrounding  mesoderm,  the  epithelium  of  the  kidney  and  of  its 
duct  alone  being  the  immediate  product  of  the  evagination.  The 
formation  of  the  collecting  tubes  and  the  uriniferous  tubules  fol- 
lows the  division  and  subdivision  of  the  compartments  into  which  the 
primitive  pelvis  and  calices  separate,  the  entire  elaborate  system  of 
tubules  resulting  from  the  extension  and  branching  of  the  primary 
canals.  The  surrounding  mesodermic  tissue  early  differentiates  a 
limiting  zone  or  primitive  capsule,  which  defines  the  form  of  the 
developing  kidney  and  opposes  the  growth  of  the  tubules  in  a  straight 
direction,  thereby  inducing  the  marked  tortuosity  accompanying  the 
subsequent  increase  in  the  length 

of  the  uriniferous  canals.     The  in-  Fig.  248. 

vagination  of  the  termination  of  the 
tubule  and  the  simultaneous  devel- 


Fig.  247. 


Sagittal  section  of  fifteen-day  rabbit  em- 
bryo :  the  developing  kidney  presents  an 
oval  mass  of  condensed  mesoderm  (m)  into 
which  the  tubular  compartments  (t)  of  the 
divided  primitive  renal  pelvis  {k)  extend. 


Sagittal  section  of  30  mm.  cow  embryo :  K, 
developing  kidney,  containing  Malpighian 
bodies  (M)  and  tubules ;  u,  part  of  renal 
pelvis  ;  W,  atrophic  Wolffian  body ;  in,  glo- 
merulus of  primary  Malpighian  body ;  /,  de- 
generating tubules  of  the  organ. 


opment  of  groups  of  capillary  blood-vessels  in  intimate  relation  with 
them  give  origin  to  the  characteristic  Malpighian  bodies.  The 
epithelium  of  all  parts  of  the  uriniferous  tubules,  of  the  renal 
pelvis,  and  of  the  ureter  is  derived  directly  from  the  outgrowth 
from  the  "Wolffian  duct ;  the  interstitial  connective  tissue,  the 
blood-vessels,  and  other  structures  are  contributed  by  the  surround- 
ing condensed  mesoderm.  Since,  as  has  been  already  stated,  the 
entire  Wolffian  body,  including  its  duct,  is  a  product  of  the  meso- 
derm, the  epithelial  evagination  and  its  derivatives  must  be  referred 
likewise  to  the  middle  blastodermic  layer,  all  parts  of  the  uririary 


205  NORMAL   HISTOLOGY. 

tract  as  far  as,  but  not  including,  the  bladder  bei?ig,  therefore,  of 
mesodermic  origin. 

The  development  of  the  urinary  bladder  is  connected  with  the 
history  of  the  allantois.  The  latter  grows  out  of  the  hind  gut  as  a 
diverticulum  which  reaches  conspicuous  dimensions,  in  many  embryos 
appearing  as  a  large,  flask-shaped  sac ;  in  man,  however,  the  allan- 
tois is  never  free,  but  grows  as  a  stalk  in  close  relations  with  the  other 
structures  passing  through  the  umbilical  opening.  The  portion  of 
the  allantoic  canal  lying  within  the  embryo  becomes  differentiated 
into  three  divisions  :  the  much  larger  middle  segment  greatly 
dilates  and  eventually  constitutes  the  bladder ;  the  outer  division, 
extending  from  the  bladder  to  the  umbilicus,  forms  the  atrophic 
urachus,  while  the  narrow  inner  portion  establishes  communication 
between  the  bladder  and  the  common  uro-intestinal  passage — the 
cloaca — and  becomes  the  urethra  proper.  The  primitive  ureter, 
which  at  first  opens  in  company  with  the  Wolffian  duct  into  the  uro- 
genital sinus,  changes  the  position  of  its  exit  until  the  tube  finally 
assumes  its  permanent  relations  and  opens  into  the  bladder.  The 
epithelium  lining  the  allantois  is  a  direct  extension  of  the  entoderm 
of  the  primary  gut ;  the  allantoic  derivatives,  including  the  bladder 
and  the  urethra,  therefore,  are  clothed  with  entodermic  cells; 
the  muscular  and  connective  tissues  of  their  walls,  however,  are  con- 
tributions from  the  mesoderm. 

The  short  female  urethra,  extending  from  the  bladder  to  the 
upper  part  of  the  vestibule,  the  representative  of  the  uro-genital 
sinus,  corresponds  with  the  primary  vesical  canal,  and  is  the  ure- 
thra proper.  In  the  male  subject  this  passage  is  supplemented 
and  greatly  lengthened  by  the  approximation  and  closure  of  the  folds 
by  which  the  sinus  is  converted  into  the  narrow  canal  extending  to 
the  end  of  the  penis.  The  male  urethra,  therefore,  consists  of  two 
morphologically  distinct  divisions :  the  urethra  proper,  which  in- 
cludes that  portion  of  the  adult  canal  lying  between  the  neck  of 
the  bladder  and  the  uterus  masculinus  or  sinus  pocularis,  this  division 
being  the  strict  homologue  of  the  female  urethra  ;  and  the  remain- 
ing part  of  the  canal,  or  supplementary  urethra,  which  represents 
the  closed  and  extended  uro-genital  sinus. 


THE   MALE   REPRODUCTIVE   ORGANS. 


207 


CHAPTER     XII. 


THE   MALE    REPRODUCTIVE    ORGANS. 


THE   TESTICLE. 

The  testicle  is  a  highly-developed  compound  tubular  gland.  The 
parenchyma  of  the  organ  is  enclosed  within  a  fibrous  capsule  of 
especial  thickness  and  strength,  the  tunica  albuginea,  which 
becomes  greatly  thickened  on  the  posterior  aspect  of  the  testicle 
to  form  a  dense  connective-tissue 
mass,  the  mediastinum,  or  the  Fig.  249. 

corpus  Highmori. 

From  the  mediastinum  stout 
fibrous  septa  radiate  to  the  pe- 
riphery, thus  dividing  the  organ 
into  a  number  of  irregular  pyram- 
idal compartments  or  lobules, 
in  which  the  seminiferous  tubules 
are  contained.  The  tunica  albu- 
ginea consists  of  a  dense  fibrous 
felt- work  of  bundles  of  fibro-elastic 
tissue  ;  the  looser,  inner  layers  sup- 
port numerous  blood-vessels,  con- 
stituting the  tunica  vasculosa. 
The  outer  surface  of  the  albuginea, 
through  the  greater  part  of  its 
extent,  is  covered  by  the  visceral 
layer  of  the  tunica  vaginalis, 
which  supplies  a  serous  investment 
to  much  of  the  testicle,  as  well  as 
to  a  portion  of  the  epididymis. 

The  testicle  lies  behind  and  out- 
side the  serous  sac,  the  latter  be- 
coming invaginated  by  the  testicle 
during  its  descent  into  the  scrotum 
der  of  the  testicle  included  between  the  reflected  folds  of  the  tunica 
vaginalis  is  devoid  of  serous  covering,  and  affords  a  position  for  the 
entrance  and  escape  of  the  blood-vessels,  the  ducts,  the  lymphatics, 
and  the  nerves. 


va 


Diagram  illustrating  the  course  and  the  rela- 
tions of  the  various  constituents  of  the  testicle 
and  the  epididymis  :  a,  tunica  albuginea ;  m, 
the  mediastinum  ;  t,  convoluted,  s,  straight,  por- 
tions of  seminiferous  tubules  ;  r,  rete  testis  ;  e, 
vasa  efferentia ;  c,  coni  vasculosi ;  te,  tube  of 
epididymis  ;  vd,  vas  deferens  ;  va,  vas  aberrans  ; 
p,  paradidymis. 


that  part  of  the  posterior  bor- 


208 


NORMAL   HISTOLOGY. 


The  seminiferous  tubules  may  be  conveniently  divided  into 
three  portions  :  (i)  the  tortuous  convoluted  tubules,  whose  wind- 
ings contribute  the  bulk  of  the  lobule,  (2)  the  straight  tubes, 
situated  in  the  apices  of  the  pyramidal  lobules,  and  (3)  the  tubules 
within  the  mediastinum,  which  by  their  union  form  the  rete 
testis. 

The  seminiferous  tubules  terminate  in  the  mediastinum,  from 
which  situation  the  seminal  canals  are  continued  by  intermediate  ves- 
sels connecting  testicle  and  epididymis  ;  these  intermediate  tubules 
are  the  vasa  efferentia  and  the  coni  vasculosi.  The  former  arise 
from  the  rete  testis,  while  the  latter  are  the  progressively  tortu- 
ous continuations  of  the  vasa  efferentia  terminating  in  a  mass,  the 

Fig.  250. 


& 


&d- 


Wm£s 


Section  of  human  testicle,  including  portion  of  tunica  albuginea,  ex- 
hibiting general  arrangement  and  structure  of  tubules  :  a,  tunica  albu- 
ginea ;  b,  seminiferous  tubules  cut  in  various  directions  ;  c,  basement- 
membrane;  d,  secreting  cells  ;  e,  groups  of  interstitial  cells  ;/,  intertubular 
connective  tissue. 


globus  major,  which  represents  the  sum  of  the  tortuous  coni  vas- 
culosi. These  last-named  canals  unite  to  form  the  main  tube  of 
the  epididymis,  which  is  sufficiently  convoluted  to  include  its 
entire  length  of  twenty  feet  within  the  inconsiderable  bulk  of  the 
epididymis. 

The  seminiferous  tubules,  130-140  /*  in  diameter,  possess  walls 
which  are  composed  of  several  layers  of  flattened  endothelioid  con- 
nective-tissue plates,  applied  to  which  a  thin  basement-membrane 


THE    MALE    REPRODUCTIVE    ORGANS. 


209 


exists  ;  inside  the  latter  lies 
cise  character  of  the  cells 
condition  of  functional 
activity  of  the  organ  ;  the 
notable  differences  distin- 
guishing the  elements  with- 
in the  resting  gland  from 
those  found  in  the  active 
organ  depend  largely  upon 
the  infrequency  within  the 
former,  and  the  almost  uni- 
versal presence  within  the 
latter,  of  cells  actively  en- 
gaged in  karyomitotic  divis- 
ion. The  different  tubules, 
however,  exhibit  great  vari- 
ation in  the  exact  stage 
of  these  changes,  adjacent 
canals,  and,  in  fact,  parts  of 
the  same  one,  often  present- 
ing the  extremes  of  the 
cycle  side  by  side. 

Next    the    basemen  t- 


the  lining  of  epithelial  cells.     The  pre- 
within    the   tubule   depends   upon   the 

Fig.  251. 


Transverse  section  of  seminiferous  tubule  from  human 
testicle  :  a,  membrana  propria ;  b,  zone  of  parietal  cells ; 
c,  mother-cells  undergoing  division  ;  d,  daughter-cells,  or 
spermatoblasts  ;  e,  partially-developed  spermatozoa ;  ff 
surrounding  intertubular  connective  tissue. 


membrane  of  the  seminiferous  tubule  lies  a  layer  of  low  cuboidal 
nucleated  parietal  cells ;  this  peripheral  zone  contains  cells  of 
two  kinds:  (1)  the  sustentacular  cells,  or  Sertoli's  columns, 
which  take  no  part  in  the  formation 
of  the  generative  elements,  and  (2) 
the  spermatogenic  cells,  which  pro- 
duce elements  intimately  related  to  the 
development  of  the  seminal  filaments. 
Inside  the  outer,  peripheral  layer,  in 
functionally  active  organs,  an  irregular 
second  zone  contains  many  elements  with 
large  transparent  nuclei  and  chromatin 
figures,  indicating  the  progress  of  cell- 
division  ;  these  are  the  mother-cells, 
the  derivatives  of  the  spermatogenic  cells 
of  the  outer  zone,  and,  in  turn,  the  parents 
of  a  numerous  progeny  of  smaller  daugh- 
ter-cells. The  nuclei  of  the  latter  con- 
stitute the  spermatoblasts,  since  it  is 

from  them  that  the  spermatozoa  are  directly  derived.      The  inner 
zone  of  the  tubule  is  frequently  occupied  by  fan-shaped  groups  of 

14 


Section  of  testicle  of  dog,  including 
part  of  seminiferous  tubule  :  a,  zone  of 
parietal  cells  containing  sustentacular 
elements  (_/") ;  b,  mother  -  cells  ;  c, 
daughter-cells  ;  d,  free  nuclei  of  sper- 
matoblasts and  developing  sperma- 
tozoa. 


210 


NORMAL    HISTOLOGY. 


developing  spermatozoa,  embedded  within  a  finely  granular,  semi- 
gelatinous  substance. 

Spermatogenesis  varies  among  the  different  classes  of  vertebrate 
animals  ;  the  account  here  given  refers  to  man  and  the  higher 
mammals. 

The  originally  round  spermatoblasts  soon  exhibit  a  tendency  to 
elongate  and  to  become  pyriform  ;  several  such  cells,  with  partially- 
formed  spermatic  filaments,  are  often  crowded  together  by  the  pressure 
of  the  surrounding  elements,  and,  in  consequence,  come  to  lie  in 
close  relation  and  in  apparent  union  with  the  centrally  projecting 
protoplasm  of  the  sustentacular  cells.     Such  appearances,  probably 


Fig.  253. 


0 


Fig.  254. 

:  ■    ds&?    ■■  -    ■■•r-v  r 


%w 


-■#,  t-  ■ 


Section  of  testicle  of  musk-rat,  ex- 
hibiting early  stage  of  spermatogenesis  : 
a,  membrana  propria ;  6,  zone  of  pari- 
etal cells  ;  c,  mother-cells  ;  d,  sperma- 
toblasts developing  into  spermatozoa. 


Section  of  testicle  of  musk-rat,  showing 
later  stage  of  spermatogenesis  :  a,  mem- 
brana propria  ;  i>,  zone  of  parietal  cells  ;  c, 
mother-cells  ;  d,  fan-shaped  masses  of  ele- 
ments concerned  in  producing  spermato- 
zoa (e). 


entirely  the  result  of  mechanical  forces,  were  formerly  regarded  as 
indicating  an  important  role  on  the  part  of  the  sustentacular  cells 
in  the  production  of  the  spermatozoa,  an  assumption  no  longer 
warranted  by  recent  investigations.  Coincidently  with  the  changes 
in  the  general  form  of  the  spermatoblasts,  the  nuclei  undergo  mod- 
ifications of  great  consequence  in  the  development  of  the  future 
spermatic  elements. 

The  views  concerning  the  genetic  relation  of  the  parts  of  the  origi- 
nal cell  to  those  of  the  resulting  spermatozoon  are  still  at  variance. 
According  to  Henle,  La  Valette  St.  George,  and  many  others,  the 
nucleus  of  the  daughter-cell  gives  rise  to  the  head,  while  from  the 
protoplasm  are  differentiated  the  middle-piece  and  the  tail.  On  the 
other  hand,  Kolliker  has  always  held,  as  recently  have  Biondi  and 
Niessing,  that  the  nucleus  undergoes  a  complicated  metamorphosis, 
producing  not  only  the  head,  but  also  the  entire  spermatozoon,  the 
protoplasm  becoming  part  of  the  granular  debris  in  which  the  groups 
of  developing  spermatozoa  lie  embedded. 


THE  MALE  REPRODUCTIVE  ORGANS. 


211 


Critical  study  of  many  specimens  convinces  the  author  that  the 
view  attributing  to  the  nucleus  the  formation  of  the  entire  sper- 
matozoon is  correct.  Without  attempting  a  detailed  account  of 
the  complicated  cycle,  these  changes  may  be  briefly  stated  to  con- 
sist in  the  increase  and  accumulation  of  the  nuclear  chromatin  in 
a  manner  resulting  in  the  differentiation  of  the  nucleus  into  two 
zones, — an  outer,  which  stains  deeply  and  is  rich  in  chromatin,  and 
an  inner,  which  appears  clear  and  is  devoid  of  chromatin.  Coinci- 
dently  with  these  changes  the  nucleus  escapes  from  the  proto- 
plasm of  the  daughter-cell  to  take  up 
FlGi==l£!  ^ts    position,    in   company   with    other 

^■V^,.  free    nuclei,    within    the    granular    re- 

mains of  the  extruded  cell-proto- 
plasm. Subsequently  the  chromatin 
becomes  especially  condensed  and  ac- 
cumulated at  the  inner  border  of  the 
darker  outer  half  of  the  nucleus ; 
from   this   zone   of  chromatin    a   deli- 


8 
ill 


Section  of  testicle  of  musk-rat,  dis- 
playing still  later  stage  of  spermato- 
genesis :  spermatozoa  are  now  well 
advanced  and  form  radially-arranged 
masses  (d) ;  other  letters  as  in  pre- 
ceding figures. 


Section  of  human  testicle,  including  parts 
of  three  tubules  (t)  and  intervening  connective 
tissue  ;  within  the  latter  lies  a  group  of  inter- 
stitial cells  (z). 


cate  projection  or  spine  grows  into,  and,  later,  through,  the  inner 
clear  half  of  the  nucleus  ;  this  outgrowth  is  the  first  indication  of 
the  future  tail  of  the  spermatic  element.  As  the  result  of  the  local- 
ization of  the  chromatin  within  the  central  part  of  the  nucleus,  the 
latter  now  exhibits  three  zones  :  an  outer  clear  cap  at  the  fore- 
pole,  a  narrow  middle  zone  filled  with  chromatin,  from  which 
the  developing  tail-fibre  extends,  and  an  inner  clear  area  which 
reaches  as  far  as  the  hind-pole  and  is  pierced  by  the  tail.  In  these 
three  nuclear  zones  the  divisions  of  the  future  mature  spermatozoon 
are  indicated :  the  outer  clear  cap  becomes  the  homogeneous  head, 
the  middle  chromatin  band  produces  the  tail  and  the  middle-piece, 


212 


NORMAL   HISTOLOGY. 


and  the  inner  clear  zone  forms  the  delicate  hyaline  envelope  invest- 
ing the  middle-piece  and  the  tail. 

Embedded  within  the  loosely  laminated  intertubular  connective 
tissue,  groups  of  polyhedral  nucleated  cells  occur  in  greater  or  less 
profusion  ;  these  interstitial  cells  are  present  within  the  testicle  of 
man  and  of  mammals  generally.  But  within  the  interstitial  tissue  of 
the  boar's  testicle  they  are  found  in  remarkable  abundance.  The 
elements,  evidently  epithelial  in  nature,  are  arranged  in  groups  or 
cylinders  in  the  interstices  between  the  seminiferous  tubules,  and 
represent  the  remains  of  the  epithelial  structures  of  the  foetal  Wolffian 
body. 

With  the  termination  of  the  convoluted  division  of  the  seminifer- 
ous tubules  the  secreting  tissue  of  the  gland  ends,  since  the  con- 
tinuation of  the  seminal  canals,  effected  by  the  straight  tubes  and 
those  forming  the  rete  testis,  represents  the  beginning  of  the  elabo- 
rate system  of  excretory  ducts  extending  from  the  testicle  to  the 
urethra. 

On  arriving  at  the  straight  canals  the  seminiferous  tubules  be- 
come reduced  in  size  (20-30  /*),  as  well  as  in  number,  the  thick 
epithelial  lining  of  the  convoluted  division  being  replaced  by  a  single 

layer  of  low  colum- 
Fig.  257.  nar  cells.     The  short 

narrow  tubuli  recti 
occupy  the  apices  of 
the  pyramidal  lobules, 
and  enter  the  medias- 
tinum, where  they  open 
into  the  irregular  canals 
of  the  rete  testis.  The 
latter  vary  in  size  from 
mere  clefts  to  channels 
approaching  in  diam- 
eter that  of  the  convo- 
luted tubules  ;  they  are 
lined  by  a  single  layer 
of  flattened  epithe- 
lial plates. 

Beginning  at  the  up- 
per end  of  the  rete  tes- 
tis, the  further  course 
of  the  seminal  canal  is 
effected  by  the  ten  to  fifteen  vasa  efferentia,  which,  by  their  pro- 
gressively increasing  convolutions,  form  as  many  conical  lobules,  the 
coni  vasculosi,  the  aggregate  of  which  makes  the  globus  major 


Section  of  tubule  of  human  epididymis :  a,  membrana  pro- 
pria ;  b,  columnar  cells  crowned  with  zone  of  long  cilia  (c) ;  d, 
layer  of  non-striped  muscle ;  e,  intertubular  areolar  tissue ;  s, 
masses  of  spermatozoa  occupying  lumen  of  tube. 


THE   MALE    REPRODUCTIVE    ORGANS. 


213 


of  the  epididymis.     The  vasa  efferentia  and  coni  vasculosi  possess 

a  stratified   columnar  epithelium,  the  inner  cells  bearing  long 

cilia ;   this   epithelium    rests  on  a 

robust    basement-membrane,   out-  Fig.  258. 

side  of  which  lies  a  fibrous  coat 

strengthened   in   many  places  by 

a     circular    layer   of    involuntary 

muscle. 

The  greatly  convoluted  tube  of 
the  epididymis  has  a  similar 
wall,  composed  of  stratified  cili- 
ated columnar  epithelium,  a 
well-marked  membrana  propria, 
augmented  by  fibrous  tissue,  and 
a  ring  of  pale  muscle  ;  this  mus- 
cular layer  gradually  thickens  on 
approaching  the  vas  deferens,  in 
whose  wall  it  becomes  a  tunic  of 
considerable  thickness. 

The  structure  of  the  spermatic 
duct,  or  vas  deferens,  closely  re- 
peats the  arrangement  of  the  tube 
of  the  epididymis.  A  stratified 
non-ciliated  columnar  epithe- 
lium, separated  from  the  tunica 
propria  by  a  well-defined  basement- 
membrane,  covers  the  mucosa; 
outside  of  the  latter  lies  a  sub- 
mucous layer  of  laminated  con- 
nective tissue,  which  is  embraced 

by  the  muscular   tunic,   consisting  of  an  inner  circular  and  an 
outer  longitudinal  layer. 

The  ampulla  possesses  the  same  coats  as  the  vas  deferens,  although 
in  the  former  the  several  layers  are  somewhat  thinner.  The  sem- 
inal vesicle,  likewise,  consists  of  a  mucous  coat,  lined  by  stratified 
columnar  epithelium,  a  submucous  and  a  muscular  tunic.  Small, 
often  branched,  tubular  glands  occur  within  the  mucous  membrane 
of  the  ampulla  and  the  seminal  vesicle.  The  ejaculatory  duct, 
formed  by  the  union  of  the  vas  deferens  and  the  duct  of  the  seminal 
vesicle,  contains  a  single  layer  of  columnar  epithelium,  supported 
by  the  fibrous  tunica  propria ;  a  thin  submucosa,  together  with  a 
slightly  developed  inner  circular  and  an  outer  longitudinal  stratum 
of  muscle,  completes  the  wall  of  the  duct. 

Connected  with  the  epididymis  are  certain  atrophic  appendages 


Section  through  lower  part  of  epididymis 
of  child,  showing  general  structure :  a,  fibro- 
serous  envelope;  b,  sections  of  convoluted  tube 
of  epididymis  ;  c,  vas  deferens  ;  d,  intertubular 
tissue ;  e,  blood-vessels. 


2I4  NORMAL    HISTOLOGY. 

which  represent  the  remains  of  fcetal  organs.  Such  structures  are 
the  paradidymis  and  the  stalked  and  sessile  hydatids. 

The  paradidymis,  or  the  organ  of  Giraldes,  consists  of  irregular 
tubules  lying  among  the  convolutions  of  the  epididymis,  which  are 
the  atrophic  remains  of  the  tubes  of  the  Wolffian  body.  They 
are  lined  with  low  columnar  or  cuboidal  epithelial  cells,  often  ciliated, 
and  are  surrounded  by  an  envelope  of  vascular  connective  tissue. 
The  tubules  of  the  paradidymis  are  usually  closed,  and  frequently 
contain  small  quantities  of  albuminous  fluid. 

The  pedunculated  or  stalked  hydatid,  common  to  both  sexes, 
probably  represents  a  part  of  the  atrophied  duct  of  the  pronephros, 
the  anterior  segment  of  the  Wolffian  body.  The  sessile  or  un- 
stalked  hydatid,  on  the  contrary,  is  limited  to  the  male  subject, 
and  is  the  slightly  expanded  proximal  end  of  the  rudimentary  Mul- 
lerian  duct.  These  sacs  are  lined  generally  by  cuboidal  cells,  and 
often  contain  a  clear  fluid. 

The  blood-vessels  of  the  testicle,  branches  of  the  spermatic 
artery,  are  distributed  to  the  mediastinum  and  to  the  loose  inner 
layer — the  tunica  vasculosa — of  the  albuginea,  including  its  pro- 
longations, the  septa.  From  the  vessels  coursing  within  these  robust 
fibrous  structures  smaller  twigs  enter  the  connective  tissue  and  pass 
between  the  individual  tubules,  around  which  they  form  rich  inter- 
tubular  capillary  net-works.  The  corresponding  veins  accom- 
pany the  arteries. 

The  lymphatics  form  a  superficial  capsular  net-work,  consisting 
of  vessels  situated  within  the  tunica  albuginea,  and  a  deeper  inter- 
tubular  plexus,  the  radicles  of  which  closely  surround  the  semi- 
niferous canals.  The  superficial  and  the  deep  lymphatics  anastomose 
to  form  within  the  mediastinum  larger  vessels,  which,  uniting  with 
those  of  the  epididymis,  constitute  one  of  the  elements  of  the 
spermatic  cord. 

Regarding  the  distribution  of  the  nerves  little  is  definitely  known 
further  than  the  penetration  of  bundles  of  mixed  fibres  between  the 
seminiferous  tubules,  around  which  they  form  plexuses  ;  the  ultimate 
termination  of  the  end-fibres  is  unknown. 

THE   SEMEN. 

The  semen  as  ejected  consists  of  the  secretion  of  the  testicle 
diluted  with  that  of  the  seminal  vesicles  and  of  the  prostate  gland, 
together  with  the  fluid  derived  from  Cowper's  glands  and  the  mucous 
membranes  traversed.  The  secretion  proper  of  the  male  sexual 
gland  consists  almost  entirely  of  spermatozoa  ;  these  latter  show  no 
movement  when  in  the  concentrated  fluid  of  the  testicle  or  epididy- 
mis :  only  after  the  dilution  normally  effected  by  the  admixture  of 


Human  spermatozoa 
as  usually  seen  :  a,  from 
the  broader  surface  ;  b, 
from  the  side ;  c,  ele- 
ment with  remains  of 
spermatoblast  adhering 
to  middle-piece. 


THE  MALE   REPRODUCTIVE   ORGANS.  2I5 

the  secretions  already  mentioned  is  it  that  the  characteristic  active 
vibratile  movements  are  observed. 

The  spermatozoa  are  minute  highly-specialized  elements,  each 
of  which  bears  at  one  end  a  long  cilium  of  exceeding  delicacy  : 
while  differing  greatly  as  to  details  of  form  and  of  size 
among  vertebrated  animals,  the  mammalian  sperma- 
tozoa possess  in  common  three  more  or  less  dis- 
tinctly defined  parts, — the  head,  the  middle-piece, 
and  the  tail. 

The  human   spermatic   filament  possesses  an 
entire  length  of  50-60  v,   of  which  the  head  con- 
tributes 3-5  11,    the   middle-piece 
4-6  p.,  while  the  remaining  43-49  y-         Fig 
belong  to  the  tail. 

The  head  varies  in  form  accord- 
ing to  the  side  examined  ;  when 
seen  on  its  broadest  surface  it  is 
egg-shaped,  the  broader  end  of 
the  head  being  directed  anteriorly, 
and  the  smaller  end  being  con- 
nected with  the  middle-piece. 
Seen  in  profile,  the  head  is  con- 
cavo-convex, and  terminates  in  a 
blunt  rounded  anterior  extremity. 

The  greatly  diminished  middle-piece  is  connected  with  the  posterior 
pole  of  the  head,  and  on  the  other  hand  fades  away  into  the  long 
delicate  caudal  filament.  After  the  action  of  certain  reagents  the 
middle-piece  splits  up  into  a  number  of  fibrillae  of  great  tenuity  (Bal- 
lowitz)  ;  in  spermatozoa  not  entirely  matured  spiral  fibrils  are  some- 
times observed  in  this  part  of  the  element.  The  centre  of  the  sper- 
matic filament  is  occupied  by  the  delicate  axial  fibre,  which  connects 
the  head  with  the  middle-piece  and  extends  through  the  middle-piece 
and  the  tail.  This  fibril  forms  the  articulation  between  the  head  and 
the  middle-piece,  and  continues  to  the  extreme  end  of  the  tail,  the 
terminal  segment  being  composed  of  the  naked  axial  fibre  alone. 
Within  the  middle-piece  the  axial  fibre  is  ensheathed  by  a  delicate 
envelope. 

The  characteristic  vibrations  of  the  spermatic  filaments  may  con- 
tinue for  a  long  time  after  ejaculation  :  when  suitably  prepared,  and 
under  favorable  conditions,  these  cells  retain  their  vitality  for  many 
hours  and  even  days.  Human  spermatozoa,  mounted  under  cover- 
glasses  and  protected  from  evaporation,  have  been  observed  by  the 
author  to  exhibit  distinct  vibratile  motion  after  the  lapse  of  over  nine 
days.     After  death  these  elements  may  continue  to  vibrate  for  forty- 


Human  spermatozoa 
highly  magnified  :  h,  m, 
t,  respectively  head, 
middle-piece,  and  tail ; 
_f,  elements  seen  from 
the  broader  surface ;  p, 
from  the  side. 


2l6 


NORMAL   HISTOLOGY. 


eight  hours,  or  longer,  within  the  fluids  of  the  seminal  tract.  Cells 
capable  of  such  tenacious  vitality  even  under  the  less  favorable  con- 
ditions outside  the  body,  exhibit  still  greater  endurance  when  aided  by 
the  favorable  conditions  for  prolonged  life  afforded  by  the  normal 
female  generative  tract  ;  in  these  organs  the  spermatozoa  no  doubt 
often  retain  their  powers  of  fecundation  for  weeks. 

These  elements  successfully  resist  the  destructive  action  of  ordi- 
nary reagents,  as  well  as  putrefactive  changes  ;  this  capability  is 
owing,  probably,  to  the  union  of  the  albuminous  with  the  calcareous 
matters,  which  latter  the  spermatozoa  contain  in  large  quantity. 

The  seminal  fluid  as  ejaculated  contains  several  constituents  recog- 
nizable by  microscopical  examination.  In  addition  to  the  sperma- 
tozoa there  are  usually  seen  spherical  or  cylindrical  masses  consisting 
of  a  clear,  hyaline,  glassy  substance  derived  from  the  seminal  vesicles  ; 
numerous  small,  pale,  delicate  granules  of  an  albuminous  nature  ;  a 
few  round  or  oval  nucleated  cells,  whose  finely  granular  protoplasm 
often  contains  fat-granules  ;  cylindrical  epithelial  cells,  and  the  char- 
acteristic prostatic  concretions  or  amyloid  bodies,  which  are  yel- 
lowish in  color,  spherical  or  triangular  in  form,  and  concentrically 
striated.  These  concretions  appear  to  be  composed  of  an  albuminous 
substance  in  combination  with  a  second  which  corresponds  to  lecithin 
(Furbringer,  Posner). 

On  standing  for  twenty-four  hours  the  semen  separates  into  an 
upper  clear  fluid  and  a  thicker,  opaque  lower  stratum  ;  the  former 
contains  few  morphological  elements,  while  in  the  lower  layer  these 
are  very  abundant.  Subsequently,  after  prolonged  standing,  two 
varieties  of  crystals  are  frequently  encountered,  those  composed 
of  ammonio-magnesium  phosphate  and  the  so-called  spermatic 
crystals.  According  to  Furbringer,  the  latter  are  formed  probably 
by  the  action  of  the  semen  on  the  prostatic  secretion  :  since  these 
crystals  are  found  almost  constantly,  after  death,  in  the  fluid  of  the 
prostate,  and  not  within  the  contents  of  the  seminal  vesicles,  they 
are  more  appropriately  termed  prostatic  crystals.  They  occur 
usually  as  prisms  or  pyramids,  colorless,  or  of  a  slight  amber  tint, 
and  break  readily  on  slight  pressure. 

THE    PENIS. 

The  penis  consists  of  three  somewhat  flattened  cylindrical  masses 
of  erectile  tissue,  the  corpora  cavernosa  and  the  corpus  spongi- 
osum, capped  by  the  conical  glans,  all  of  which  are  held  together 
by  connective  tissue  and  enveloped  by  the  skin  and  subcutaneous 
tissue. 

The  two  cavernous  bodies  are  enclosed  within  a  stout  fibrous 
envelope,  the  tunica  albuginea,  which  reaches  a  thickness  of  about 


THE   MALE    REPRODUCTIVE    ORGANS. 


217 


i  mm. ,  and  is  composed  of  closely  interwoven  longitudinal  bundles 
of  white  fibrous  tissue,  intermingled  with  well-developed  elastic  fibres. 
Within  this  common  in- 
vestment  each    body   is  Fig.  261. 
surrounded  by  an  indi- 
vidual sheath  of  circu- 
larly-disposed    bundles, 
which,   in  the   mid-line, 
takes    part    in    forming 
the  pectinate  septum  ;  in 
other  places  the  sheaths 
contribute  the  trabecular 
belonging  to  the  enclosed 
erectile  tissue. 

The  trabecular  spring 
from  all  parts  of  the  in- 
vesting   fibrous    tunics, 
including    the     septum, 
and   pass   inward,   join- 
ing with  their  fellows  on 
all  sides  to  form  a  frame- 
work,   the  basis  of  the 
cavernous  tissue,  which 
occupies  the  entire  cav- 
ernous body.     While  the  trabecular  are  stouter  and  larger  near  the 
periphery  than  in  the  centre,  the  included  spaces,  on  the  contrary,  are 
larger   near   the   middle   and 
smaller  at  the   circumference 
of  the  cavernous  bodies ;  to- 
wards the  anterior  end  of  the 
penis  the  spaces  become  gen- 
erally larger.     In  addition  to 
the  white  fibrous  tissue  com- 
posing their  principal  part,  the 
trabecular  contain  elastic  fibres 
and    unstriped    muscles,    to- 
gether with  the  blood-vessels 
which  the  larger  bands  sup- 
port.    The  interspaces  of  this 
spongy  structure  are  cavern- 
ous venous  channels,  which 

form  an  intercommunicating  system  of  canals  throughout  the  cav- 
ernous body.  These  spaces,  lined  with  endothelium,  and  communi- 
cating on  the  one  hand  with  the  arteries  and  on  the  other  with 


Section  of  penis  of  child  near  end  :  a,  corpora  cavernosa  ; 
b,  fibrous  envelope  of  same  ;  c,  imperfect  septum  ;  d,  corpus 
spongiosum  ;  e,  urethra  ;  f,  sebaceous  glands  ;  g,  epithelium 
of  skin  ;  i,  that  lining  the  sac  of  prepuce  ;  h,  section  of  latter ; 
k,  blood-vessels. 


Fig.  262. 


Section  of  erectile  tissue  of  human  penis  :  z/,  blood- 
spaces  lined  with  endothelium;  t,  fibrous  trabecule 
containing  bundles  of  non-striped  muscle  (tn)  cut  in 
various  directions  ;  i,  blood-vessels. 


2I3  NORMAL  HISTOLOGY. 

the  veins,  during  erection  become  enormously  distended,  with  a  cor- 
responding reduction  in  the  thickness  of  the  intervening  trabecule. 

The  corpus  spongiosum  in  its  structure  resembles  closely  the 
cavernous  bodies,  being  limited  by  a  fibrous  tunic  from  which  spring 
the  trabecular  of  the  cavernous  tissue  enclosing  the  venous  spaces. 
The  fibrous  envelope  is  less  developed  than  in  the  case  of  the 
cavernous  bodies,  while  the  proportion  of  elastic  fibres  is  greater, 
peculiarities  resulting  in  less  unyielding  rigidity  in  this  part  of  the 
penis  during  erection.  The  fibrous  trabeculae  of  the  spongy  body 
are  thinner  but  more  uniform  in  diameter,  and  the  enclosed  spaces 
possess  greater  similarity  in  size,  although  somewhat  smaller  than  the 
corresponding  channels  of  the  corpora  cavernosa ;  their  long  axis 
generally  coincides  with  that  of  the  penis.  The  erectile  tissue  of 
the  corpus  spongiosum  is  continued  into  the  glans,  the  spaces,  how- 
ever, becoming  somewhat  reduced  and  provided  with  finer  trabeculae. 
Immediately  around  the  urethra  a  zone  of  condensed  fibrous  tissue 
intermingled  with  a  quantity  of  unstriped  muscle  occurs,  in  addition 
to  which  a  small  amount  of  muscular  tissue  frequently  exists  within 
the  fibrous  tunic  of  the  spongy  body,  as  well  as  within  the  larger 
trabeculae. 

The  smaller  divisions  of  the  arteries  of  the  cavernous  bodies, 
branches  of  the  internal  pudic,  are  supported  by  the  larger  bands  of 
fibrous  tissue ;  from  these  situations  the  arteries  pass  into  the  capil- 
lary vessels,  which,  as  a  rule,  communicate  with  the  blood-spaces  of 
the  erectile  tissue ;  these  spaces,  in  turn,  are  drained  by  the  venous 
radicles,  which  empty  into  veins  escaping  at  the  roots  of  the  penis 
or  into  the  dorsal  vein.  Not  all  the  capillaries,  however,  open  into 
the  cavernous  spaces,  since  those  destined  for  the  nutrition  of  the 
tissues  at  once  terminate  in  the  veins,  thus  establishing  a  direct  cir- 
culation, which  forms  the  chief  course  of  the  blood  during  the  passive 
condition  of  the  penis.  As  a  compensative  provision  for  the  great 
expansion  of  the  trabeculae  during  erection,  the  arterioles  are  often 
so  long  that  they  present  marked  tortuosity,  sometimes  protruding 
as  twists  and  loops  into  the  undistended  cavernous  spaces  ;  in  recog- 
nition of  this  peculiarity  these  vessels  have  been  named  the  helicine 
arteries. 

In  addition  to  the  usual  channel  of  the  blood  into  the  spaces  by 
means  of  the  capillary  vessels,  a  direct  communication  exists  between 
the  arterioles  and  the  larger  spaces  at  the  circumference  of  the  cavern- 
ous bodies  (Langer).  The  arrangement  of  the  vascular  supply  of 
the  corpus  spongiosium  and  of  the  glans  is  identical  with  that  above 
described,  all  the  blood,  however,  being  conveyed  into  the  spaces 
through  the  capillaries. 

The  masses  of  erectile  tissue,  enclosed  within  their  respective 


THE  MALE  REPRODUCTIVE  ORGANS. 


219 


sheaths,  are  enveloped  in  the  general  areolar  tissue  supporting  the 
larger  blood-vessels,  nerves,  and  lymphatics,  the  whole  being  covered 
in  by  the  investing  integument.  The  skin  of  the  penis  is  attached 
over  its  body  by  loose  subcutaneous  tissue,  allowing  of  free  move- 
ment and  great  distention  ;  it  is  distinguished  by  its  dark  color,  thin- 
ness, freedom  from  fat,  and,  throughout  the  greater  part  of  its  ex- 
tent, absence  of  hairs.  At  the  margin  of  the  prepuce  the  skin  as- 
sumes the  character  of  a  true  mucous  membrane,  becoming  delicate, 
rosy,  and  moist ;  the  base  of  the  glans  is  generously  supplied  with 
modified  sebaceous  follicles,  the  glands  of  Tyson,  sometimes  called 
glandulse  odoriferae,  on  account  of  their  peculiar  secretion  ;  par- 
tially inspissated  accumulations  of  the  latter,  together  with  abraded 
epithelial  scales,  constitute  the  smegma.  Upon  the  glans  the  in- 
tegument is  very  intimately  and  immovably  united  to  the  fibrous 
tunic  of  the  spongy  tissue,  and  contains  large  papillae  in  which 
rich  vascular  loops  and  special  nerve-endings  are  situated;  the 
skin  in  this  situation  is  free  from  glands. 

The  lymphatics  of  the  penis  consist  of  a  superficial  and  a  deep 
set :  the  former  extends  beneath  the  integument  as  a  subcutaneous 
net-work,  whose  principal  vessels  accompany  the  larger  blood-vessels 
in  their  course  and  terminate  in  the  superficial  inguinal  glands,  while 
the  latter  passes  from  the  cavernous  and  spongy  bodies,  along 
with  the  deep  veins,  to  the  deep  lymphatic  glands  within  the  pelvis. 
The  lymphatics  begin  in  the  interfascicular  clefts  within  the  larger 
trabeculae  and  the  dense  fibrous  laminae  which  constitute  the  sheaths 
of  the  erectile  masses  ;  delicate  radicles  continue  the  lymph-channels 
from  the  clefts  to  the  larger  lymphatic  vessels. 

The  nerves  of  the  penis  include  trunks  derived  both  from  the 
cerebro-spinal  and  from  the  sympathetic  system,  those  from  the  latter 
being  contributed  by  the  hypogastric  plexus  ;  the  sympathetic 
fibres  are  distributed  entirely  to  the  erectile  tissue  of  the  cavernous 
and  spongy  bodies.  The  sensory  and  motor  nerves  are  ob- 
tained from  the  dorsal  and  superficial  perineal  branches  of  the  pudic 
nerve,  and  terminate  within  the  skin  and  the  mucous  membrane. 
Special  nerve-endings,  represented  by  numerous  examples  of  the 
simple  and  compound  genital  corpuscles,  as  well  as  by  the  cor- 
puscles of  Vater,  are  found  in  the  integument  of  the  glans  and  of 
other  parts  of  the  penis  :  the  structure  of  these  peculiar  bodies  has 
been  considered  in  Chapter  VI. 

THE   PROSTATE   GLAND. 

The  prostate  body  is  a  compound  tubular  gland.  The  outer 
surface  of  the  organ  is  invested  by  a  stout  fibrous  covering, 
the   continuation  of  the  contiguous   fascia,  beneath  which   lies  an 


220 


NORMAL   HISTOLOGY. 


Fig.  263. 


inner  envelope  of  involuntary  muscle.  From  the  latter  muscu- 
lar septa  penetrate  in  all  directions  between  the  acini  of  glandular 
tissue ;  immediately  surrounding  the  urethra  a  thick  muscular  layer 
also  exists. 

The  prostatic  acini  may  be  regarded  as  highly  developed  ure- 
thral glands,  which  they  closely  resemble,  opening  by  a  dozen  or 

more  ducts  on  the  free  sur- 
face of  the  urethra.  Of 
these  ducts  two  of  especial 
size  empty  on  either  side 
of  the  urethral  crest,  and, 
repeatedly  subdividing, 
communicate  with  the 
numerous  closely  -  packed 
acini  constituting  the  cen- 
tral lobe.  The  other  di- 
visions of  the  gland  are 
simpler  in  structure,  since 
they  contain  tubular  al- 
veoli, much  less  closely 
placed,  which  open  into  a 
slightly  wavy  duct. 

The  epithelium  lining 
the  alveoli  is  short  colum- 
nar in  character,  and  fre- 
quently possesses  more 
than  a  single  row  of  cells, 
smaller  spherical  or  pyri- 
form  elements  filling  up 
the  interstices  between  the 
outer  ends  of  the  somewhat  tapering  cells  next  the  lumen.  The 
nuclei  of  the  epithelial  elements  are  situated  eccentrically,  lying 
nearer  the  ends  of  the  cells  directed  towards  the  basement-mem- 
brane. These  cells  in  elderly  subjects  not  infrequently  contain 
pigment. 

In  addition  to  the  fibrous  and  elastic  connective  tissue  among  the 
acini  of  the  gland,  bundles  of  involuntary  muscle  pass  in  all  di- 
rections between  the  alveoli,  and  in  many  places  constitute  almost 
the  entire  tissue  separating  the  adjacent  acini.  While  present  in  all 
parts  of  the  gland,  the  quantity  of  muscle  varies  in  different  parts  of 
the  organ  ;  it  is  poorest  in  the  central  lobe,  where  the  acini  are  best 
developed,  and  richest  in  the  upper  part  of  the  posterior  post-ure- 
thral  division  and  in  the  extreme  fore  part  of  the  organ.  In  the  low- 
est part  of  the  posterior  segment  the  involuntary  muscle  is  supple- 


Section  of  human  prostate,  exhibiting  general  disposition 
of  acini  :  a,  fibrous  envelope  ;  b,  groups  of  tubular  acini ; 
c,  sections  of  prostatic  ducts ;  e,  interacinous  fibro-mus- 
cular  tissue. 


THE   MALE   REPRODUCTIVE   ORGANS. 


221 


mented  by  connective  tissue  in  forming  the  interalveolar  partitions. 
The  layer  of  involuntary  muscle  surrounding  the  urethra  is  continu- 
ous behind  with  the  vesical  sphincter,  and  in  front  with  the  muscular 
envelope  of  the  membranous  portion  of  the  canal. 

On  either  side  of  the  urethral  crest,  which  occupies  the  posterior 
surface  of  the  prostatic  portion,  a  depression  marks  the  position  of 
the  prostatic  sinus,  into  which  open  the  orifices  of  the  twelve  to 
twenty  prostatic  ducts.  These  recesses  are  lined  with  a  continuation 
of  the  stratified  squamous  epithelium  which  covers  the  adjacent 
urethral  mucous  membrane  ;  these  cells,  however,  are  soon  replaced 

Fig.  264. 


Section  of  human  prostate  more  highly  magnified  :  a,  some  of  the  tubular  acini 
lined  with  columnar  cells ;  b,  muscular  tissue  of  the  intertubular  septa. 


within  the  ducts  by  others  of  the  columnar  type.  As  has  already 
been  pointed  out,  the  sinus  pocularis,  or  uterus  masculinus, 
occupying  the  anterior  part  of  the  urethral  crest,  is  to  be  regarded 
as  homologous  with  the  cavity  of  the  vagina  and  the  uterus,  the  layer 
of  involuntary  muscle  belonging  to  the  especial  wall  of  the  divertic- 
ulum corresponding  to  the  uterine  muscular  tissue,  while  the  small 
tubular  glands  present  within  the  mucous  membrane  lining  the  sinus 
are  the  homologues  of  those  of  the  uterus.  The  prostate  itself, 
which  is  developed  as  a  thickening  of  the  urinary  tract,  cannot  be 
regarded  in  any  sense  as  homologous  with  the  uterus,  notwithstand- 
ing the  apparently  close  relations  with  the  sinus  pocularis,  since  these 
relations  are  secondary  and  attained  in  the  course  of  its  subsequent 
growth. 

The  blood-vessels  of  the  prostate  gland,  branches  of  the  adjacent 
vesical,  hemorrhoidal,  and  pudic  arteries,  pass  into  the  interior  of  the 
organ  within  the  larger  connective-tissue  septa,  where  they  break  into 


222 


NORMAL   HISTOLOGY. 


smaller  twigs,  which  follow  the  ducts  into  the  lobules,  the  capillary- 
vessels  then  forming  net-works  about  the  individual  alveoli.  The 
veins  on  emerging  from  the  deeper  parts  of  the  gland  form  a  rich 
plexus  within  the  fibrous  envelope  about  the  base  and  sides  of  the 
organ. 

The  lymphatics  originate  within  the  connective-tissue  septa  as 
interfascicular  clefts  ;  these  unite  with  definite  channels,  which,  in 
turn,  form  the  larger  lymphatic  trunks  accompanying  the  veins  in 
their  course  to  the  neighboring  deep  lymph-glands. 

The  nerves,  derived  principally  from  the  hypogastric  plexus,  are 
composed  of  both  medullated  and  non-medullated  fibres,  and  pass 
along  the  stouter  connective-tissue  trabecular  towards  the  glandular 
compartments  ;  their  ultimate  mode  of  termination  is  still  uncertain. 
Corpuscles  of  Vater  have  also  been  observed  along  the  course  of  the 
more  superficial  nerve-trunks. 

The  secretion  of  the  prostate  gland — the  prostatic  fluid — is 
a  thin,  opalescent,  slightly  acid  liquid,  usually  containing  epithelial 
cells  and  granules.  The  dilution  of  the  secretion  of  the  testicle 
seems  to  be  an  important  use  of  this  fluid  ;  and  when  so  mixed,  on 
standing  for  some  time  the  thin  rhombic  prostatic  or  Charcot's 
crystals  make  their  appearance. 

Within  the  ducts  or  acini  of  the  prostate  gland  additional  small, 
irregularly  round,  laminated  bodies,  the  prostatic  concretions,  often 
occur  ;  these  are  constant  in  advanced  age,  but  they  are  found  often 
also  in  young  subjects  ;  these  accumulations  seem  to  be  albumino- 
calcareous  in  nature  and  present  a  concentric  lamination. 

THE    GLANDS    OF    COWPER. 

Cowper's  glands  are  two  small  racemose  structures,  whose 
rounded,  somewhat  flattened  masses,  10—13  mm.  in  diameter,  lie  be- 
neath the  anterior  part  of  the  membranous  urethra.  Each  gland 
is  composed  of  several  small  lobes,  which  pour  out  their  secretion 
through  the  long  excretory  duct  into  the  posterior  part  of  the  bulbous 
portion  of  the  urethra,  where  a  minute  orifice  marks  the  termination 
of  the  tube.  The  lobules  composing  the  gland  are  held  together, 
as  well  as  enveloped,  by  a  common  investment  of  fibrous  connective 
tissue  containing  some  involuntary  muscle. 

The  acini  are  occupied  by  clear  low  cylindrical  cells,  resembling 
in  character  and  in  secretion  those  of  a  mucous  gland.  The  epithe- 
lium lining  the  small  ducts,  into  which  the  acini  directly  open, 
consists  of  elements  cuboidal  in  form  ;  these  cells  are  gradually 
replaced  by  taller  columnar  ones  as  the  urethra  is  approached. 
In  addition  to  the  epithelium  and  delicate  connective  tissue,  the 
walls  of  the  ducts  are  strengthened  by  bundles  of  unstriped  muscle. 


THE   MALE   REPRODUCTIVE    ORGANS.  22X 

Cowper's  glands  secrete  a  clear,  viscid  fluid,  regarding  the  use  of 
which  little  is  known  with  certainty. 

The  blood-vessels  supplying  Cowper's  glands,  derived  usually 
as  branches  of  the  artery  of  the  bulb,  pass  between  the  lobules,  in 
company  with  the  ducts,  supported  by  the  intervening  connective 
tissue  ;  the  capillaries  form  net-works  around  the  individual  acini. 
Lymph-spaces  occur  within  the  fibrous  envelope  and  within  the 
larger  masses  of  connective  tissue  penetrating  the  organ. 

The  nerves  are  branches  from  the  pudic  nerve  :  regarding  their 
termination  little  is  definitely  known. 


224 


NORMAL   HISTOLOGY. 


CHAPTER    XIII. 

THE  FEMALE  REPRODUCTIVE  ORGANS. 
THE  OVARY. 

The  ovary  is  attached  to  the  posterior  surface  of  the  broad  liga- 
ment along  its  shorter  straight  border,  the  sides  and  convex  edge  of 
its  flattened  oval  mass  being  invested  by  the  serous  covering  contin- 
uous with  the  peritoneum  of  the  adjacent  surfaces.  The  serous 
membrane  reflected  over  the  organ  is  modified  both  in  appearance 
and  in  structure,  since  the  usual  shining  smoothness  of  its  surface 
is  replaced  by  dulness,  and  the  flat  endothelial  plates  are  supplanted 


Section  of  ovary  of  cat :  C,  cortex  containing  peripheral  zone  of  Graafian 
follicles  (g~)  in  various  stages  of  development  ;  c,  well-advanced  follicle  ex- 
hibiting ovum  (o),  discus  proligerus  (d),  and  membrana  granulosa  (m)  ; 
c",  c" ,  other  large  follicles,  from  which  ova  are  absent ;  k,  peripheral  section 
of  large  follicle  which  membrana  granulosa  seemingly  fills;  s,  ovarian 
stroma  ;  /,  corpus  luteum  ;  M,  medulla  containing  many  vascular  channels  (i). 

by  the  low  columnar  cells  which  constitute  the  germinal  epithelium. 
The  transition  of  the  latter  into  the  usual  peritoneal  covering  is  indi- 
cated by  a  distinct  demarcation  around  the  attachment  of  the  ovary. 
The  ovary  is  divided  into  two  parts,  the  cortex  and  the  medulla, 
the  boundaries  of  which  are  somewhat  conventional  and  by  no  means 
sharply  defined.  The  cortex  includes  the  peripheral  zone,  contain- 
ing the  Graafian  follicles  and  the  ova,  and  occupies  approximately 
the  outer  third  of  the  organ,  while  the  medulla  embraces  the  re- 


THE  FEMALE  REPRODUCTIVE  ORGANS. 


225 


Fig.  266. 


6«i§g§s 


d& 


maining  central  portions  of  the  ovary,  in  which  the  blood-vessels  are 
conspicuous  constituents. 

The  bulk  of  the  organ  consists  principally  of  the  stroma,  together 
with  the  contained  blood-vessels  and  the  Graafian  follicles. 

The  ovarian  stroma  is  a  peculiar  form  of  connective  tissue  dis- 
tinguished by  the  great  number  of  its  spindle-cells,  which,  while  dis- 
tributed through  all  parts  of  the  organ,  are  especially  closely  packed 
in  the  cortex,  particularly  near  the  periphery. 

The  cortical  stroma,  arranged  in  variously-directed  bundles,  is 
greatly  condensed  immediately  beneath  the  germinal  epithelium,  the 
zone  of  condensed  tissue  appear- 
ing as  a  distinct  peripheral  layer, 
the  so-called  tunica  albuginea  ; 
the  latter,  however,  does  not  rep- 
resent an  independent  structure,  as 
does  the  sheath  bearing  the  same 
name  in  the  testicle,  but  only  a 
peripheral  band  of  the  stroma  of 
especial  density. 

The  most  important  constit- 
uents of  the  cortex  are  the 
Graafian  follicles,  which  are 
exclusively  limited  to  this  part 
of  the  ovary,  where  thev  occur 
in  all  stages  of  development. 
The  youngest  and  least  matured 
Graafian  follicles  are  plentifully 
scattered  through  the  outer  part 
of  the  cortex,  where  in  many  ani- 
mals, as  the  cat  and  the  rabbit, 
they  form  almost  a  complete 
zone.  The  most  immature  fol- 
licle consists  of  the  ovum  sur- 
rounded by  a  single  layer  of  flat- 
tened cells,  the  progenitors  of  the 

membrana  granulosa,  outside  of  which  lie  the  cells  of  the  general 
stroma,  without  the  intervention  of  a  special  limiting  membrane. 
Among  the  immature  follicles  are  others  in  various  stages  of  more 
advanced  development,  in  which  the  ovum  is  embraced  by  two  or 
more  rows  of  polygonal  cells  ;  around  such  ova  the  stroma  is  con- 
centrically disposed,  a  condition  foreshadowing  the  membrana  gran- 
ulosa and  the  theca  of  later  stages.  The  cells  which  surround  the 
ovum  by  their  division  give  rise  to  the  numerous  elements  lining  the 
follicle;  they  were  originally  derived  from  the  germinal  epithelium 


.  .-■,  _'.-■' 


h 


Section  of  human  ovary,  including  cortex  :  a, 
germinal  epithelium  of  free  surface ;  b,  tunica 
albuginea ;  c,  peripheral  stroma  containing  im- 
mature Graafian  follicles  (d) ;  e,  well-advanced 
follicle  from  whose  wall  membrana  granulosa 
has  partially  separated ;  f,  cavity  of  liquor 
folliculi ;  g,  Oram  surrounded  by  cell-mass  con- 
stituting discus  proligerus. 


226 


NORMAL   HISTOLOGY. 


as  cylindrical  masses  which  penetrate  the  stroma  and  undergo  prolif- 
eration.    With  the  increase  in  size  which  accompanies  their  devel- 
opment   the    Graafian 
Fig.  267. 


68SBB& 

Mr 


'.V  •■  ^'' 


follicles  pass  towards  the 
inner  limits  of  the  cortex 
bordering  on  the  me- 
dulla, where  they  un- 
dergo further  enlarge- 
ment ;  after  a  time  their 
diameter  includes  al- 
most the  entire  cortex, 
and  extends  from  the 
medulla  to  the  surface 
of  the  ovary.  Subse- 
quently the  position  of 
the  follicle  becomes  evi- 
dent as  a  distinct  pro- 
jection on  the  free 
surface,  marking  the 
point  at  which  the  final 
rupture  of  the  sac  and 
the  escape  of  the  ovum 
take  place.  Such  dis- 
charge usually  coincides 
with  the  phenomena  at- 
tending menstruation. 

The  mature  Graaf- 
ian follicles  appear  as 
clear  vesicles,  4-8  mm. 
in  diameter,  and,  on 
section,  exhibit  a  char- 
acteristic arrangement.  The  follicle  is  defined  from  the  surrounding 
tissue  by  a  condensed  layer  of  stroma,  which  forms  a  sheath,  the 
theca  folliculi ;  this  envelope  is  composed  of  two  layers,  an  outer 
tunica  fibrosa,  containing  fibrous  connective  tissue  and  coarser 
blood-vessels,  and  an  inner  tunica  propria,  rich  in  cells,  small 
blood-vessels,  and  capillaries.  Within  the  theca  follows  the  mem- 
brana  granulosa,  consisting  of  many  layers  of  small  polyhedral 
epithelial  cells,  the  descendants  of  the  single  row  of  original  cells 
contained  within  the  young  follicle ;  at  one  point  the  membrana 
granulosa  presents  a  thickening,  which  is  continued  as  a  zone  of 
cells  immediately  surrounding  the  ovum  ;  this  constitutes  the  discus 
proligerus,  which  remains  in  contact  with  the  ovum  after  its  escape. 
The  cells  of  the  discus  which  lie  next  the  ovum  are  placed  vertically 


Section  of  cortex  of  cat's  ovary,  exhibiting  large  Graafian  folli- 
cle :  a,  peripheral  zone  of  condensed  stroma  ;  b,  groups  of  im- 
mature follicles  ;  c,  theca  of  follicle  ;  d,  membrana  granulosa  ; 
e,  discus  proligerus  ;  f,  zona  pellucida  ;  g,  vitellus  ;  h,  germinal 
vesicle  ;  i,  germinal  spot ;  k,  cavity  of  liquor  folliculi. 


THE    FEMALE    REPRODUCTIVE    ORGANS. 


227 


to  its  surface,  forming  a  radial  zone,  the  corona  radiata.  The 
interior  of  the  follicle  is  occupied  by  an  albuminous  fluid,  the 
liquor  folliculi,  derived  probably  as  an  exudate  from  the  blood- 
vessels of  the  theca,  as  well  as  from  the  breaking  down  of  some  of 
the  central  cells  of  the  follicle. 

Within  the  discus  proligerus  lies  the  ovum,  a  spherical  body  about 
.2  mm.  in  diameter,  enclosed  within  a  distinct  membrane,  the  zona 
pellucida,  which  presents  a  delicate  radial  striation.  These  mark- 
ings are  regarded  by  many  as  due  to  the  presence  of  fine  canals, 
which   may  facilitate  the  access 


Fig.  268. 


■^ 


b'«\ 


-d 


Ovum  from  ovary  of  cat  :  d,  innermost  cells  of 
discus  proligerus,  between  which  processes  from 
zona  pellucida  (z)  extend  ;  m,  vitelline  membrane  ; 
v,  vitellus  ;  g,  germinal  vesicle  ;  s,  germinal  spot. 


of  fluids  and  possibly,  also,  of 
the  spermatozoa  to  the  contained 
cell.  The  zona  pellucida  is  a 
protecting  membrane,  derived 
from  the  cells  of  the  surrounding 
discus  proligerus,  and  does  not, 
strictly  considered,  constitute  a 
part  of  the  ovum  proper,  since 
it  lies  outside  of  the  true  cell- 
wall,  the  vitelline  membrane. 

The  protoplasm  of  the  ovum, 
or  vitellus,  occupies  almost  the 
entire  area  within  the  zona  pellu- 
cida, and  is  limited  by  the  delicate 
and  inconspicuous  vitelline  mem- 
brane, which  closely  approxi- 
mates the  inner  surface  of  the 
zona  pellucida.  The  protoplasm  of  the  ovum  is  modified  by  the 
presence  of  numberless  particles  of  fatty  matter  which  lie  embedded 
within  the  albuminous  protoplasm  proper.  The  germinal  vesicle, 
corresponding  to  the  nucleus  of  the  ovum,  is  situated  eccentrically, 
limited  by  a  distinct  membrane,  and  contains  the  germinal  spot  or 
nucleolus.  Of  the  parts  of  the  ovum,  the  germinal  vesicle  is  the 
most  important,  since  in  it,  as  in  the  nucleus  of  cells  in  general,  are 
inaugurated  the  important  changes  attendant  upon  the  phenomena 
of  cell  division.  The  threads  of  chromatin  form  a  loose,  irregular 
net-work  throughout  the  germinal  vesicle,  the  interspaces  of  which  are 
filled  with  a  substance  representing  the  nuclear  juice.  While  each 
Graafian  follicle  contains,  as  a  rule,  but  a  single  ovum,  exceptions 
are  observed  occasionally  where  two,  and  even  three,  ova  are  found 
within  the  same  vesicle. 

The  formation  of  new  follicles  continues  for  only  a  short  time 
after  birth  ;  ovisacs  are  then  most  numerous,  the  entire  number 
contained  within  the  two  ovaries  of  the  child  being  estimated  at  over 


223  NORMAL   HISTOLOGY. 

seventy  thousand.  In  view  of  the  unquestionably  large  number  of 
follicles  in  very  young  ovaries,  and  the  relatively  small  proportion  of 
ova  which  reach  maturity,  the  degeneration  of  many  follicles, 
after  attaining  a  certain  development,  seems  certain;  the  atrophic 
remains  of  such  degenerating  Graafian  vesicles,  continually  encoun- 
tered, point  conclusively  to  the  fate  of  a  large  contingent. 

The  medulla  contrasts  with  the  cortex  by  its  looser  structure  and 
the  number  and  the  size  of  its  vascular  canals.  The  stroma  of  this 
portion  of  the   ovary  more   nearly  resembles   ordinary  connective 

tissue,  the  peculiar  spindle-cells  occur- 
Fig.  269.  ring  much  less  abundantly,  while  the 

fibrous  tissue  forms  an  important  con- 
stituent of  the  supporting  matrix.  A 
considerable  amount  of  involuntary 
muscle  is  mixed  throughout  the  fibrous 
bundles  separating  and  surrounding  the 
numerous  blood-vessels.  The  latter 
are  larq-elv  venous,  the  laree  sinus - 
like    veins    being    very   conspicuous 

Section  of  medulla  of  human  ovary  :  .  111 

avascular    canals    surrounded    by   the       ODjeCtS  111  the  medulla. 

stroma-ceiis  and  the  connective  tissue;         In  addition  to  the  elements  already 

w,   group    of    interstitial    cells    derived         .  ..        ,  -  ,  -     . 

from  wolffian  tubules.  described,  groups  of  polygonal  inter- 

stitial cells  occur  between  the  bun- 
dles of  stroma-tissue,  especially  in  the  medulla,  but  also  within  the 
cortex.  These  cells  are  epithelial  in  character,  and  represent  the 
remains  of  the  cylindrical  cell-masses  which  grow  from  the  Wolffian 
body  into  the  tissue  of  the  primitive  ovary.  In  some  animals  the 
interstitial  cells  are  much  more  numerous  than  in  the  human  ovary  ; 
in  the  rabbit  these  cells  constitute  an  important  part  of  the  stroma 
of  the  organ. 

On  the  escape  of  the  ovum,  the  ruptured  and  partly-collapsed 
follicle  becomes  filled  with  the  blood  poured  out  from  the  torn  vessels 
of  the  walls  of  the  vesicle  ;  subsequent  changes  lead  to  the  conver- 
sion of  the  follicle  into  a  corpus  luteum.  The  production  of  these 
characteristic  bodies  depends  principally  upon  the  proliferation  of 
the  walls  of  the  follicle,  in  some  cases  the  interstitial  cells  being  in- 
volved; the  process  results  in  the  plication  of  the  remains  of  the 
envelope,  as  well  as  in  the  gradual  formation  of  a  mass  of  polyhedral 
cells,  between  which  the  capillaries  derived  from  the  vessels  of  the 
follicle  extend  ;  the  enclosed  area  corresponds  to  the  remains  of  the 
cavity  of  the  follicle,  and  is  for  a  time  occupied  with  the  yellowish 
mass  composed  of  the  degenerating  blood-clot  and  the  membrana 
granulosa  ;  these  tissues  are  replaced  by  a  shrunken  fibrous  area, 
which  is  later  invaded  by  the  proliferating  peripheral  cells.     When 


THE    FEMALE    REPRODUCTIVE    ORGANS. 


229 


Fig. 


Portion  of  well-developed  corpus  luteum  from 
ovary  of  rabbit  :  a,  polyhedral  cells  separated 
by  vascular  connective  tissue;  b,  blood-vessel. 


best  developed,  the  corpus  luteum  is  sharply  defined  from  the  sur- 
rounding stroma,  and  in  appearance  recalls  somewhat  the  liver,  the 
polygonal  cells  being  surrounded  by  capillary  blood-vessels.  Subse- 
quent changes  lead  to  retrogression 
and  disappearance  of  the  cells,  the 
entire  mass  becoming  fibrous  and 
cicatricial  in  character,  but  remain- 
ing visible  for  many  months  as 
an  obscure,  shrunken,  irregularly- 
plicated  body  in  the  midst  of  the 
cortical  tissues.  While  the  forma- 
tion of  a  corpus  luteum  follows  the 
discharge  of  every  mature  ovum, 
when  such  escape  is  followed  by 
pregnancy  the  yellow  body  be- 
comes exceptionally  large  and  well 
developed,  presenting  a  large  round 
mass,  2-3  cm.  in  diameter,  which 
retains  its  distinctive  character  much 
more  tenaciously  than  the  corpus 

of  ordinary  menstruation.  These  differences  led  to  the  distinction 
of  the  corpus  luteum  of  pregnancy  as  the  true  yellow  body  as 
contrasted  with  the  ordinary  or  false  ;  the  former  large  symmetri- 
cally-developed body  has  been  regarded  as  positive  proof  of  preg- 
nancy, a  conclusion,  however,  which  the  repeated  observation  of 
identical  bodies  in  the  ovaries  of  virgins  by  no  means  upholds  :  the 
evidence  afforded  by  such  corpora  lutea  should  be  regarded  as  cor- 
roborative rather  than  as  positive. 

The  blood-vessels  of  the  ovary  enter  at  the  hilus  along  the 
attached  border.  They  directly  penetrate  to  the  medulla,  smaller 
twigs  passing  to  supply  the  cortex  and  the  Graafian  vesicles.  Each 
of  these  sacs  is  surrounded  by  a  net-work  of  vessels,  especially  con- 
spicuous in  the  larger  follicles.  The  venous  vessels  within  the 
medulla  are  of  large  size,  the  channels  resembling  sinuses  in  their 
tortuous  course  and  thin  walls. 

The  lymphatics  are  numerous  within  the  medulla,  while  their 
terminal  radicles  have  been  traced  within  the  cortex  to  the  cleft- 
like spaces  within  the  fibrous  tunic  of  the  walls  of  the  larger 
follicles. 

The  nerves  of  the  ovary  include  medullated  and  pale  fibres,  repre- 
senting both  the  cerebro-spinal  and  the  sympathetic  system.  After 
passing  into  the  interior  of  the  organ,  fine  twigs  enter  the  cortex, 
where  they  have  been  traced  into  the  envelope  of  the  larger  Graafian 
follicles. 


230  NORMAL   HISTOLOGY. 


THE    PAROVARIUM. 

The  parovarium,  the  epobphoron,  or  the  organ  of  Rosen- 
miiller,  consists  of  a  group  of  tubular  structures  lying  transversely 
within  the  broad  ligament,  between  the  ovary  and  the  oviduct;  the 
short  vertical  tubules  lie  irregularly  parallel  or  converge  somewhat 
at  their  ovarian  ends,  while  their  opposite  extremities  are  connected 
with  a  longitudinal  head-tube  of  larger  diameter,  which  extends 
downward  often  for  some  distance  within  the  broad  ligament.  The 
tubules  are  lined  by  low  columnar  epithelial  cells,  the  represen- 
tatives of  the  elements  clothing  the  embryonic  canals.  The  parova- 
rium represents  the  partially-obliterated  re- 
Fig.  271.  mains  of  parts  of  the  Wolffian  body;  the 

transverse  canals  correspond  to  the  tubules  of 
the   body,    while   the    head-tube    is    identical 
with   the   upper   part  of  the  Wolffian   duct; 
when  this  latter  canal  persists  throughout  the 
%K   c  i  I  -i     greater  part  of  its  original  extent  it  constitutes 
It/yVW^      Gartner's   duct,   the  homologue  of  the  vas 
w    gfb  A',s      deferens.    Other  fcetal  remains  are  sometimes 
Portion  of  tubules  of  par-    encountered    as   rudimentary  tubules  em- 

ovarium :      ?v,    canals     lined  ■* 

with    cuboidai     epithelium     bedded  within  the  broad  ligament  nearer  the 

embedded    within    surround-       uterus    than    the  parovarium  .    these   structures 
ing  connective  tissue  (s).  .  x 

constitute  the  paroophoron,  and  represent 
the  atrophic  transverse  tubules  of  the  lower  portion  of  the  Wolffian 
body,  being  homologous  with  the  paradidymis  in  the  male.  The 
closed  tubules  of  the  paroophoron  are  lined  by  low  columnar  epi- 
thelium, and  are  often  occluded  by  the  partially-shed  cells. 

The  stalked  hydatid  of  Morgagni  frequently  forms  a  conspicuous 
appendage  to  the  ovary.  This  pedunculated  vesicle  represents  the 
remains  of  the  duct  of  the  pronephros,  and  is  common  to  both  sexes  ; 
low  columnar  or  cuboidai  epithelium  forms  the  lining  of  its  dilated 
sac  and  stalk  as  far  as  pervious. 

THE   OVIDUCT. 

The  oviduct,  or  Fallopian  tube,  consists  of  three  coats, — an 
inner  mucous,  a  middle  muscular,  and  an  outer  serous. 

The  mucous  membrane  of  the  oviduct  is  thrown  into  longitu- 
dinal folds,  which  correspond  in  their  amplitude  to  the  general 
variation  in  the  size  of  the  tube,  being  low  towards  the  small  uterine 
end  and  increasing  in  height  and  in  complexity  on  approaching  the 
expanded  fimbriated  extremity  of  the  canal.  On  transverse  section 
through  the  smaller  portions  of  the  tube,  the  longitudinal  folds  give 
to  the  lumen  a  generally  stellate  outline,  the  complexity  of  the  figure 


THE  FEMALE  REPRODUCTIVE  ORGANS. 


231 


mucous 


^ 


increasing  as  the  sections  approach  the  fimbria,  owing  to  the  addition 

of  numerous  secondary  plications  which  there  exist.     The 

membrane     of     the 

oviduct  consists  of  a  FlG'  2"2' 

fibro-elastic     tunica 

propria  and  a  single 

layer   of   columnar 

ciliated    epithelial 

cells,  whose  ciliary 

wave    sweeps     from 

the    fimbria  towards 

the   uterine   end   of 

the  tube.     All  parts 

of  the  canal  are  lined 
with  ciliated  cells,  in- 
cluding the  inner  sur- 
face of  its  expanded 
ovarian  end;  at  the 
free  edge  of  the  lat- 
ter the  ciliated  co- 
lumnar cells  of  the 
tubal  surface  are  re- 
placed by  the  flat 
endothelial  plates  of  the  peritoneum  which  invests  the  outer  aspect 
of  the  tube. 

The  outer  layers  of  the  tubal  mucous  membrane  contain  scattered 
longitudinal  bundles  of  involuntary  muscle,  which  represent  a 
poorly-developed  muscularis  mucosae  ;  outside  these  a  thin  layer  of 
fibrous  connective  tissue  answers  to  a  submucosa,  and  contains  the 
larger  blood-vessels  and  the  lymphatics.  Glands  are  absent  within 
the  mucous  membrane  of  the  oviduct. 

The  muscular  tunic  consists  of  a  principal  inner  circular  layer 
of  non-striped  muscle  and  a  slightly  developed  outer  layer  consist- 
ing of  an  incomplete  zone  of  longitudinal  bundles. 

The  serous  coat  consists  of  the  fibro-elastic  stroma  and  the  endo- 
thelial plates  of  the  general  peritoneum. 

The  blood-vessels  of  the  oviduct  are  branches  from  the  ovarian 
and  uterine  arteries  and  the  corresponding  veins  ;  the  arteries  pos- 
sess a  tortuous  course  and  extend  along  the  bases  of  the  folds  of 
the  mucosa  ;  from  these  vessels  smaller  twigs  arise,  which  break  up 
into  the  capillary  net -works  destined  for  the  various  coats. 

The  larger  lymphatics  accompany  the  blood-vessels  and  commu- 
nicate with  the  lymph-spaces  within  the  deeper  layers  of  the  mucosa. 
The  nerves,  derived  from  the  ovarian  and  uterine  plexuses,  con- 


Section  of  human  oviduct  near  fimbria ;  a,  lumen  of  tube  en- 
croached upon  by  complicated  folds  ;  b,  layer  of  ciliated  columnar 
epithelium  ;  c,  fibrous  tissue  supporting  plications  ;  d,  circular  layer 
of  muscle ;  e,  longitudinal  bundles  of  muscle-cells  \f,  external  fibrous 
tissue. 


232 


NORMAL   HISTOLOGY. 


sist  of  both  medullated  and  pale  fibres ;  the  principal  trunks  run  in 
company  with  the  blood-vessels  as  far  as  the  mucous  membrane ; 
their  ultimate  distribution  and  mode  of  termination  are  still  un- 
certain. 

THE   UTERUS. 

The  uterus  being  the  fused  morphological  continuations  of  the 
oviducts,  similarity  approaching  identity  in  the  structure  of  the  two 
segments  of  the  original  tube  is  to  be  expected  ;  this  resemblance,  in 


Fig.  273. 


Section  of  human  uterus,  including  mucosa  (a)  and  adjacent  muscular  tissue  {i)  ;  c,  epithelium  of 
free  surface  and  tubular  uterine  glands  (d)  ;  /,  deepest  layer  of  mucosa,  containing  fundi  of  glands  ; 
k,  strands  of  non-striped  muscle  penetrating  within  the  mucosa. 

fact,  exists.  The  uterus  is  composed  of  a  mucous,  a  muscular,  and 
a  serous  coat,  modified  to  meet  the  demands  of  special  functions. 

The  mucosa,  1-2  mm.  in  thickness,  consists  of  a  tunica  propria 
formed  of  delicate  bundles  of  fibrous  tissue,  intermingled  with  some 
elastic  fibres  and  many  leucocytes,  and  the  epithelium.  The  latter 
is  a  single  layer  of  ciliated  columnar  cells,  whose  ciliary  current 
is  directed  towards  the  cervix.  The  tunica  propria  contains  numer- 
ous slightly  wavy  tubular  uterine  glands,  limited  by  a  delicate 
basement-membrane  and  lined  by  an  extension  of  the  ciliated  colum- 
nar epithelium  of  the  adjacent  mucous  surface.  Since  a  submucosa 
is  wanting  in  the  uterine  wall,  the  blind  and  often  forked  extremities 
of  the  glands  abut  directly  upon  the  muscular  tissue. 

The  mucosa  of  the  uterine  cervix  differs  materially  from  that 
of  the  body  of  the  organ,  being  thicker  and  firmer,  and  within  the 
lower  third  beset  with  minute  papillae  covered  with  stratified  squa- 
mous epithelium.     In  the  upper  half  or  two-thirds  of  the  cervix 


THE   FEMALE   REPRODUCTIVE   ORGANS.  2XX 

the  epithelium  is  ciliated  columnar,  similar  to  that  of  the  body  of  the 
organ.  In  addition  to  the  scattered  tubular  follicles,  the  representa- 
tives of  the  usual  uterine  glands,  numerous  short  mucous  crypts, 
with  expanded  blind  extremities,  lie  embedded  within  the  mucosa  ; 
these  pour  out  the  thick  glairy  mucous  secretion  which  is  character- 
istic of  the  glands  of  the  cervix.  Not  infrequently  retention  of  the 
secretion  takes  place  in  some  of  these  mucous  follicles,  the  glands 
then  undergoing  transformation  into  greatly-distended  cysts,  the 
ovula  Nabothi;  these  appear  as  translucent  yellowish  vesicles  em- 
bedded within  the  mucosa  and  readily  seen  by  the  unaided  eye.     In 

Fig.  274. 


Section  of  uterus  through  lower  segment  of  cervix  from  child :  a,  vaginal  surface  covered  with 
squamous  epithelium;  b,  c,  d,  e,  variously-dis posed  bundles  of  non-striped  muscle ;  f,  g,  blood-ves- 
sels ;  k,  fibrous  tunica  propria  covered  by  columnar  epithelium  (z)  ;  k,  folds  of  mucosa  projecting 
within  lumen  of  canal  (C). 

the  absence  of  glands  the  mucous  membrane  of  the  lowest  part  of 
the  cervix  still  further  resembles  that  of  the  adjacent  vaginal  surface. 
The  exterior  of  the  projecting  portion  of  the  cervix  is  covered  with 
an  extension  of  the  vaginal  mucous  membrane.  With  the  recurrence 
of  each  menstrual  period  the  uterine  mucous  membrane  under- 
goes changes  destined  to  prepare  this  surface  as  a  favorable  place 
for  the  reception  and  retention  of  the  ovum  during  gestation  in  the 
event  of  impregnation.  Greatly-increased  vascularity,  softening  and 
thickening  of  the  mucous  membrane,  with  increase  in  the  length  of 
the  glands  and  in  the  number  of  the  leucocytes,  are  among  the 
changes  then  taking  place.     Should  impregnation  occur,  these  altera- 


2^4  NORMAL   HISTOLOGY. 

tions  become  more  pronounced  and  result  in  the  formation  of  the 
decidua.  When  incidental  merely  to  the  phenomena  of  menstrua- 
tion, the  flow  of  blood  following  the  rupture  of  the  over-distended 
capillaries  is  accompanied  by  a  degeneration  of  the  inner  portions 
of  the  uterine  mucous  membrane,  including  the  glands,  which  are 
cast  off  as  far  as  the  deepest  layers  next  the  muscular  tissue;  from 
this  external  zone  of  remaining  unimpaired  tissue  the  regeneration 
of  the  mucous  membrane  proceeds. 

The  muscular  coat  of  the  uterus  consists  of  bundles  of  involun- 
tary muscle  separated  by  bands  of  connective  tissue  and  surrounding 
numerous  vascular,  especially  venous,  channels.  While  more  or  less 
irregularly  arranged,  the  muscular  tissue  is  disposed  in  three  general 
strata,  an  inner,  a  middle,  and  an  outer  layer.  The  inner  layer, 
upon  which  directly  rests  the  mucosa,  is  often  regarded  as  belonging 
to  the  mucous  membrane,  being  in  fact  the  hypertrophied  muscu- 
laris  mucosae  ;  it  is  composed  principally  of  irregular  longitudinal 
or  oblique  bundles,  and  contributes  about  1.5  mm.  of  the  entire 
muscular  tunic.  The  middle  layer  is  the  most  robust,  forming  the 
greater  part  of  the  muscular  wall,  consisting  chiefly  of  bundles  having 
a  general  circular  disposition.  This  layer  is  also  distinguished  by  the 
numerous  large  venous  channels  enclosed  between  its  bundles; 
hence  the  name,  stratum  vasculare.  The  outer  layer  of  muscle, 
about  .  1  mm.  in  thickness,  is  made  up  partly  of  circular  and  partly 
of  longitudinal  bundles,  the  latter  predominating  and  being  closely 
related  to  the  overlying  serous  coat.  Many  bundles  of  this  outer 
layer  pass  obliquely  across  the  fundus  and  into  the  broad  ligament ; 
some  of  these  enter  the  round  ligaments  and  accompany  the  areolar 
tissue  and  the  blood-vessels  composing  these  structures  towards  the 
groin,  while  others  extend  along  the  oviducts  ;  strong  muscular  bands 
also  run  from  the  uterus  into  the  ovarian  ligaments.  The  muscu- 
lature of  the  cervix  is  characterized  by  greater  regularity  in  its 
arrangement,  a  distinct  inner  longitudinal,  a  middle  circular,  and  an 
outer  longitudinal  layer  being  present.  During  pregnancy  the 
muscular  tissue  of  the  uterus  becomes  enormously  increased,  the 
augmentation  depending  not  only  upon  the  excessive  size  of  the 
already  existing  individual  muscle-cells,  but  also  upon  the  appear- 
ance of  additional  new  muscle-cells. 

The  serous  coat  of  the  uterus  is  composed  of  the  usual  constitu- 
ents of  the  peritoneum,  the  fibro-elastic  stroma  being  covered  by  the 
outer  sheet  of  endothelium. 

The  blood-vessels  supplying  the  uterus  are  very  numerous.  The 
arteries,  branches  from  the  ovarian  and  the  uterine,  pass  beneath  the 
serous  coat  into  the  muscular  tunic,  where  many  twigs  are  given  off 
for  distribution  to  the  tissue  of  this  layer  ;  the  capillary  vessels  pass 


THE   FEMALE    REPRODUCTIVE   ORGANS.  23  C 

between  and  into  the  muscle-bundles  supported  by  the  intervening 
connective  tissue  ;  the  terminal  branches  reach  the  mucosa,  where 
they  break  up  into  capillaries,  which  form  net-works  around  the 
uterine  glands  and  beneath  the  free  surface.  In  addition  to  the 
trunks  accompanying  the  principal  arteries,  the  veins  contribute 
numerous  channels  to  the  middle  muscular  coat,  in  which  they  form 
a  plexiform  system  of  thin-walled  sinus-like  blood-spaces  within 
the  intermuscular  connective  tissue. 

The  lymphatics  are  represented  within  the  uterine  mucosa  by  a 
wide-meshed  net-work  of  canals  within  the  deeper  layers  of  the  tunic, 
as  well  as  by  blind,  slightly  club-shaped  branches  and  the  interfas- 
cicular lymph-spaces.  Within  the  muscular  tunic  lymphatic  channels 
occur  among  the  muscle-bundles,  particularly  of  the  middle  layer  ; 
these  unite  with  the  larger  lymphatics  lying  within  the  subserous 
tissue. 

The  nerves  supplying  the  uterus  are  derived  from  the  inferior 
hypogastric  and  the  ovarian  plexus,  together  with  branches  from  the 
lower  sacral  nerves  ;  they  consist,  therefore,  of  both  medullated  and 
non-medullated  fibres  :  minute  ganglia  have  been  observed  in  con- 
nection with  the  latter.  The  larger  trunks  send  many  twigs  to  the 
muscular  tissue  ;  the  final  termination  of  the  branches  passing  into 
the  mucosa  is  still  undetermined. 

THE   VAGINA. 

The  walls  of  the  vagina  consist  of  a  mucous  membrane,  a 
muscular  coat,  and  a  fibrous  adventitia. 

The  mucous  membrane  is  covered  with  a  thick  stratified 
squamous  epithelium,  which  rests  upon  a  tunica  propria  rich  in 
elastic  fibres  and  leucocytes  and  beset  with  numerous  papillae  ;  the 
latter,  when  small,  do  not  impress  the  free  surface,  the  epithelium 
presenting  an  uninterrupted  plane.  Larger  elevations,  however, 
occur  as  the  prominent  folds  constituting  the  rugae,  which  include 
within  their  structure  not  only  the  tissues  of  the  mucosa  but  also 
bundles  of  involuntary  muscle  and  numerous  large  veins,  these 
latter  bestowing  upon  the  parts  somewhat  the  character  of  cavernous 
tissue.  Leucocytes  are  plentifully  scattered  within  the  mucosa  of 
the  entire  vaginal  tract,  but  in  certain  places,  particularly  in  the 
anterior  wall  near  the  orifice  of  the  vagina,  these  cells  are  very  nu- 
merous, and  give  the  mucosa  the  appearance  of  adenoid  tissue  ; 
solitary  lymph-follicles  also  exist.  True  glands  are  not  found  in 
the  vaginal  mucous  membrane  ;  the  watery  acid  secretion  bathing  its 
surface  seems  to  be  the  product  of  the  general  mucosa.  The  hymen 
consists  of  a  crescentic  or  circular  duplicature  of  the  mucous  mem- 
brane strengthened  by  an  intervening  layer  of  fibrous  tissue. 


2^5  NORMAL   HISTOLOGY. 

The  deepest  part  of  the  mucosa  is  continuous  with  the  loosely- 
woven,  highly-vascular  submucous  tissue ;  outside  the  latter  follows 
the  muscular  tunic,  composed  of  an  inner  circular  and  an  outer 
longitudinal  stratum  of  involuntary  muscle.  These  layers  are  not 
sharply  defined,  but  are  blended  with  one  another  by  numerous 
oblique  bundles.  The  outer  adventitious  coat  consists  of  a  dense 
fibrous  tunic,  rich  in  elastic  tissue,  which  contributes  greatly  to  the 
strength  of  the  vaginal  walls  :  this  fibrous  coat  is  best  developed  in 
the  anterior  wall  of  the  canal,  where  it  closely  unites  the  vagina  to  the 
bladder,  and  encloses  within  its  firm,  compact  mass  the  urethra. 

The  blood-vessels  of  the  vagina  are  very  numerous  ;  the  larger 
twigs  break  up  within  the  submucous  tissue  into  smaller  vessels, 
which  pass  to  supply  the  muscular  coat  and  the  mucous  membrane. 
Those  entering  the  latter  terminate  in  capillary  loops  lying  beneath 
the  epithelium  and  within  the  papillae.  The  veins  correspond  with 
the  larger  arteries,  but,  in  addition,  form  dense  plexiform  net- 
works beneath  the  serous  coat.  Around  the  entrance  of  the  vagina 
the  number  and  size  of  the  venous  channels  give  the  submucous 
coat  the  character  of  cavernous  tissue. 

The  lymphatics  form  net-works  within  the  mucosa  and  muscularis, 
which  unite  with  the  larger  lymph-channels  within  the  adventitia. 

The  nerves  supplying  the  vagina,  derived  from  the  hypogastric 
plexus  and  from  the  sacral  and  pudic  nerves,  consist  of  both  pale 
and  medullated  fibres.  Numerous  microscopic  ganglia  occur  in  con- 
nection with  the  sympathetic  fibres.  Special  end-bulbs,  or  the 
genital  corpuscles  of  Krause,  exist  within  the  vaginal  mucosa.' 

THE   GENITALIA. 

The  labia  majora  consist  of  the  folded  integument  enclosing  an 
abundance  of  adipose  tissue,  together  with  blood-vessels,  nerves, 
glands,  and  bundles  of  involuntary  muscle  ;  their  outer  surface 
corresponds  to  the  usual  integument  of  the  vicinity,  while  internally 
they  assume  partly  the  character  of  the  adjacent  mucous  membrane. 
The  median  surfaces  of  the  labia  contain  little  fat,  but,  on  the 
other  hand,  many  bundles  of  elastic  and  unstriped  muscular  tissue. 
Sebaceous  follicles  and  sweat-glands  are  numerous,  but  they 
are  more  plentiful  on  the  outer  than  on  the  inner  surface  of  the  labia. 
Owing  to  the  unusual  quantity  of  pigment  contained  within  the 
deeper  layers  of  the  epithelium,  the  labial  integument  is  especially 
dark. 

The  labia  minora,  or  nymphae,  include  between  their  folds  of 
mucous  membrane  vascular  areolar  tissue ;  their  external  surfaces 
resemble  somewhat  in  appearance  the  adjoining  integument  of 
the   external   labia,  with  which   they   are   continuous.      Vascular 


THE   FExMALE    REPRODUCTIVE   ORGANS. 


237 


papillae  and  well-developed  sebaceous  follicles  are  common  to 
both  surfaces  of  the  nymphae,  but  sweat-glands,  hairs,  and  fat  are 
wanting.  The  interior  of  the  nymphae  contains  venous  spaces  in 
abundance,  which,  in  connection  with  the  unstriped  muscle  also 
present,  produce  a  layer  resembling  erectile  tissue.  The  blood- 
vessels of  the  labia  majora  are  similar  to  those  supplying  the  integu- 
ment ;  in  the  nymphae  the  mucous  surfaces  are  beset  with  vascular 
papillae,  which  contain  the  terminal  capillary  loops. 

The  lymphatics  consist  of  the  interfibrillar  lymph-clefts  and  the 
more  definite  channels  which  are  present  as  small  lymphatic  vessels 
accompanying  the  larger  blood-vessels  from  the  areolar  tissue. 

The  nerves  of  the  nymphae,  derived  from  branches  supplying  the 
lower  part  of  the  vagina,  include  both  medullated  and  pale  fibres  : 
numerous  special  end-bulbs,  the  genital  corpuscles  of  Krause, 
represent  the  particular  terminations. 

The  clitoris  largely  repeats  the  structure  of  the  corresponding 
male  organ,  subject,  however,  to  the  modifications  incident  to  the 
feebler  development  of  the  parts.  The  glans  possesses  small  and 
large  papillae,  which  contain  simple  and  compound  arterial  tufts, 
while  some  of  the  smaller  elevations  are  occupied  by  the  peculiar 
nervous  end-bulbs  or  the  genital  corpuscles.  Sebaceous  follicles 
also  surround  the  glans  and  are  present  in  the  outer  layer  of  the 
prepuce  ;  on  the  glans  itself  they  are  almost  wanting.  The  erectile 
tissue  constituting  the  diminutive  corpora  cavernosa  and  the  glans 
consists  of  the  same  elements  as  the  corresponding  structures  of  the 
penis. 

The  mucous  membrane  lining  the  vestibule  closely  resembles  that 
covering  the  inner  surface  of  the  nymphae,  and  is  prolonged  inward 
into  the  vagina  and  the  urethra.  A  thick  layer  of  stratified  squa- 
mous epithelium  rests  upon  a  tunica  propria  containing  bundles 
of  elastic  tissue,  and  many  mucous  follicles,  the  latter  being  espe- 
cially abundant  in  the  vicinity  of  the  urethral  orifice.  The  submucous 
tissue  around  the  vestibule  and  base  of  the  nymphae  is  so  generously 
supplied  with  intercommunicating  venous  channels  that  in  many 
places  the  part  assumes  the  characters  of  erectile  tissue. 

The  glands  of  Bartholin  are  two  round  or  oval  yellowish  bodies, 
about  1  cm.  in  diameter,  lying  on  either  side  of  the  lower  part  of 
the  vagina.  These  structures  are  the  homologues  of  Cowper'  s  glands 
in  the  male  ;  they  are  racemose  glands,  composed  of  small  groups 
of  acini  lined  with  clear  mucous  cells.  Each  gland  is  connected 
with  the  inner  surface  of  the  nymphae  by  a  long  slender  duct  lined 
with  low  cuboidal  epithelium.  The  character  of  the  secretion  of 
these  glands  is  muco-serous. 

The  female  urethra  differs  from  the  canal  in  the  male  in  being 


2*8 


NORMAL   HISTOLOGY. 


short,  very  dilatable,  and  of  large  size.  Its  walls  consist  of  a  mucosa, 
composed  of  fibrous  tissue  intermingled  with  many  elastic  fibres  and 
containing  large  numbers  of  leucocytes,  and  an  epithelium  of  the 
stratified  squamous  variety,  continuous  with  the  transitional  epi- 
thelium of  the  bladder  on  the  one  hand  and  with  the  epithelium  of 
the  vestibule  on  the  other.  The  mucous  membrane  of  the  urethra 
presents  longitudinal  folds,  especially  on  the  posterior  wall,  and  con- 
tains many  tubular  mucous  glands  ;  several  of  these,  near  the 
urethral  orifice,  are  of  large  size.  Near  the  vestibular  end  of  the 
canal  the  mucosa  contains  so  many  leucocytes  that  the  membrane 
resembles  diffuse  adenoid  tissue.  Outside  the  submucosa  follows 
involuntary  muscle,  disposed  as  an  inner  thin  longitudinal  and 
an  outer  thick  circular  layer. 


Fig 


THE    MAMMARY    GLANDS. 

The  mammary  glands  are  usually  described  in  connection  with  the 
female  reproductive  organs,  although  these  structures  are  only  modi- 
fied and  specialized  sebaceous  integumentary  glands ;  strictly  re- 
garded, the  mammae,  therefore,  belong  to  the  consideration  of  the  skin. 
Each  mamma  consists  of  from  fifteen  to  twenty  distinct  tubo- 
racemose  glands  or  lobes,  which  are  held  together  by  connective 
tissue  and  united  into  a  single  hemispherical  mass  by  adipose  tissue, 
which  fills  all  irregularities  and  interspaces  between  the  divisions  of 
the  organ.  Each  lobe,  supplied  by  its  own  excretory  duct,  is  sub- 
divided by  penetrating  fibrous  septa  into  lobules,  which,  in  turn,  are 
composed  of  groups  of  individual  acini.     The  histological  details  of 

the  secreting  portions  of 
the  organ  vary  with  the 
stages  of  its  functional  ac- 
tivity: the  following  de- 
scription applies  to  the 
active  glands  as  seen 
during  lactation. 

The  acini,  usually  tu- 
bular or  saccular  in  form, 
are  grouped  together  to 
constitute  the  lobules; 
limited  by  a  distinct 
membrana  propria, 
they  are  lined  by  a  single 
layer  of  short  columnar 
or  polyhedral  epithe- 
lial cells,  whose  protoplasm  differs  in  appearance  with  the  condition 
of  secretion.     At  rest,  these  cells  are  uniformly  granular;  as  secre- 


Section  of  human  mammary  gland  during  lactation  :  a,  a, 
sections  of  the  large  tubular  acini  which  constitute  almost  the 
entire  lobule;   b,  interlobular  connective-tissue  septa. 


THE  FEMALE  REPRODUCTIVE  ORGANS. 


239 


Fig.  276. 


tion  advances,  their  granular  protoplasm  becomes  broken  up  and 
displaced  by  the  accumulation  of  oil-globules  within  the  cell ;  these 
minute  oil-drops  exist  at  first  as  minute  separate  particles,  which 
gradually  increase  in  size,  until  finally  they  become  confluent  and 
form  a  single  large  globule,  which  occupies  the  greater  part  of  the 
entire  cell ;  the  nucleus  in  consequence  is  displaced  towards  the 
periphery,  next  the  basement- membrane,  where  it  lies  embedded 
within  the  thin  belt  of  protoplasm  occupying  the  outer  zone  of  the 
cell.  During  secretion  the  acini  possess  a  comparatively  wide 
lumen,  since  the  epithelial  layer  forms  but  a  narrow  lining  to  the 
irregular  spherical  or  tubular  spaces.  The  cells  within  a  single  acinus 
often  contain  very  unequal  amounts  of  oil ;  some  of  the  elements 
are  so  loaded  that  the  entire  cell  is  occupied  by  the  oil-drop,  while, 
on  the  other  hand,  the  neigh- 
boring cells  may  contain  so 
little  oil  that  the  presence  of 
the  fatty  particles  is  masked 
by  the  general  protoplasm. 
Between  the  extremes  all  gra- 
dations may  be  found.  Upon 
reaching  a  certain  tension,  the 
contained  oil-globules,  es- 
caping in  the  direction  of  least 
resistance,  are  discharged  into 
the  cavity  of  the  acinus,  where 
they,  together  with  the  gran- 
ular debris  of  old  epithelial 
cells,  are  collected  within  an 
albuminous  fluid  and  consti- 
tute the  lactiferous  secre- 
tion. The  assumed  destruction  of  the  epithelial  cells  following  the 
discharge  of  the  oil-globules  is  improbable,  since  the  cell  then  simply 
enters  upon  a  period  of  rest  and  repair,  during  which  its  powers  of 
secretion  are  recuperated.  In  the  earliest  stage  of  the  activity  of  the 
mammary  gland,  when  the  flow  of  milk  is  first  established,  the  acini, 
in  many  cases,  still  retain  their  primitive  condition  of  solidity ; 
while  the  cells  at  the  periphery  remain  as  the  secreting  elements, 
those  occupying  the  centre  of  the  acinus  undergo  fatty  degeneration, 
some  become  disintegrated,  while  others  are  cast  off  as  masses  which 
constitute  the  colostrum-corpuscles  found  in  the  milk  during  the 
first  few  days. 

The  secretion  accumulated  within  the  comparatively  large  alveoli 
is  carried  off  by  the  terminal  branches  of  the  ducts,  whose  walls 
consist  of  a  basement-membrane  and  a  single  layer  of  low  colum- 


Section  of  human  mammary  gland,  including  sev- 
eral acini  («)  engaged  in  sluggish  secretion  of  milk  ; 
b,  epithelial  elements  containing  oil-droplets  ;  c,  inter- 
acinous  connective  tissue. 


240 


NORMAL   HISTOLOGY. 


nar  or  flattened  epithelium.  The  large  excretory  canals,  the 
galactophorous  ducts,  each  of  which  collects  the  secretion  from 
an  entire  lobe,  pass  as  distinct  tubes  to  the  nipple  ;  they  possess  walls 
of  considerable  thickness,  composed  of  fibrous  and  elastic  connective 
tissue,  together  with  some  unstriped  muscle  derived  from  the  nipple. 
The  lining  epithelial  cells  are  columnar  to  within  a  few  millimetres 
of  the  external  openings  of  ducts,  where  the  epithelium  becomes 
stratified  and  continuous  with  the  epidermis.  The  fifteen  to  twenty 
excretory  ducts,  after  a  longer  or  shorter  course,  converge  towards 
the  areola,  within  whose  area  they  undergo  considerable  dilatation 
to  form  the  ampullae,  which  serve  during  lactation  as  temporary 
reservoirs  for  the  milk. 

The  nipple  consists,  in  addition  to  the  external  covering  of  pig- 
mented and  greatly  wrinkled  skin  which  is  perforated  at  the  tip  by 
the  openings  of  the  excretory  milk-ducts,  of  a  central  mass  composed 
of  the  lactiferous  canals  and  the  blood-vessels  embedded  within 
the  fibro-elastic  tissue.  A  considerable  amount  of  unstriped  mus- 
cle exists,  disposed  as  encircling  and  radiating  fibres  ;  upon  the 
contraction  of  this  muscle,  which  responds  to  mechanical  stimulus, 
the  erectility  of  the  nipple  principally  depends.  The  cutaneous 
papillae  are  supplied  with  numerous  nerve-terminations,  which  insure 
a  high  degree  of  sensitiveness.     The  subcutaneous  tissue  of  the 

nipple  proper  contains 
no  fat;  around  its  base 
and  over  the  areola 
elevations  mark  the 
orifices  of  the  scat- 
tered groups  of  little 
racemose  structures 
which  constitute  the 
glands  of  Mont- 
gomery. The  in- 
tegument of  the 
areola  surrounding 
the  base  of  the  nipple 
usually  possesses  con- 
siderable pigment, 
the  amount  greatly  in- 
creasing during  preg- 
nancy ;  large  sweat- 
glands  and  numerous  well-developed  sebaceous  follicles  are  also 
present  within  this  area. 

The  relative  proportion  of  the  glandular  structure  to  the  inter- 
vening connective  tissue  varies  with  the  condition  of  functional  activity 


Section  of  human  mammary  gland  undergoing  retrogressive 
changes  after  lactation  :  a,  sections  of  ducts ;  b,  atrophic  acini ; 
c,  fat-cells  ;  d,  interlobular  connective  tissue. 


THE  FEMALE  REPRODUCTIVE  ORGANS.         2Al 

of  the  organ.  During  lactation  the  secreting  tissue  predominates, 
and  the  septa  are  reduced  to  mere  partitions  ;  before  pregnancy 
has  taken  place  the  connective  tissue  and  fat  form  the  bulk  of  the 
organ,  the  glandular  structures  then  being  represented  by  the  system 
of  ducts  and  excretory  tubes,  since  the  acini  are  present  only  as 
small  solid  rudimentary  cylindrical  cell-masses.  After  lactation 
the  secreting  parts  of  the  organ  atrophy  and  become  much  less  con- 
spicuous, some  of  the  acini  almost  entirely  disappearing,  while  con- 
nective tissue  and  fat  constitute  the  greater  part  of  the  mamma. 
The  termination  of  the  period  of  sexual  activity  is  followed  by  the 
permanent  atrophy  of  the  gland-tissue,  which  finally  is  almost 
complete,  the  entire  mamma  being  then  composed  of  connective 
tissue  and  fat,  with  scarcely  a  trace  of  the  former  conspicuous  se- 
creting structures.  The  rudimentary  breasts  of  children  of  both 
sexes,  and  of  the  adult  male,  contain  principally  connective  tissue, 
in  which  excretory  ducts,  attached  to  small  groups  of  immature 
acini,  lie  embedded.  Under  exceptional  circumstances  the  male 
mammary  gland  may  secrete  true  milk. 

The  principal  blood-vessels  supplying  the  mammary  glands  run 
mostly  in  the  superficial  tissues  somewhat  radially  towards  the  areola ; 
from  these  vessels  on  the  anterior  surface  of  the  organ  branches 
penetrate  into  the  glandular  mass  and  pass  between  the  lobules, 
giving  off  twigs  which  break  up  into  capillaries  enclosing  the  alveoli. 
The  cutaneous  papilla?  are  supplied  with  capillary  tufts  where  not 
occupied  by  nervous  structures.  The  vascular  supply  of  the  nipple, 
while  generous,  does  not  include  cavernous  tissue,  the  erectility  of 
this  part  being  due  largely  to  the  muscle. 

The  lymphatics  include  the  radicles  enclosed  within  the  fibrous 
septa  of  the  gland,  as  well  as  a  net-work  of  subcutaneous  lymph- 
canals  within  the  more  superficial  portions  of  the  organ.  The 
lymphatic  vessels  are  closely  related  to  the  surrounding  chain  of 
lymphatic  glands,  as  well  as  to  those  within  the  axilla. 

The  nerves  are  distributed  more  richly  to  the  superficial,  cutaneous 
parts  of  the  organ  than  to  its  secreting  tissue.  They  are  principally 
medullated  fibres,  those  supplying  the  papillae  of  the  nipple  and  the 
areola  in  many  cases  ending  in  special  tactile  corpuscles  ;  the 
nerves  entering  the  base  of  the  nipple  often  bear  corpuscles  of  Vater. 
The  deeper  parts  of  the  glands  receive  principally  the  pale  fibres 
destined  for  the  control  of  the  blood-vessels,  not,  however,  to  the 
exclusion  of  medullated  fibres  ;  ganglion-cells  have  also  been  ob- 
served in  connection  with  the  latter. 

Milk  is  composed  microscopically  of  a  clear  fluid,  the  milk- 
plasma,  in  which  numbers  of  small  oil-globules,  2-5  fi  in  diam- 
eter, together  with  the  granular  debris  of  disintegrated  cells,  are 

16 


242 


NORMAL    HISTOLOGY. 


Fig.  27S 
A 


B 


°f^°0®h 


Human  milk:  A,  usual  appearance; 
B,  shortly  after  delivery  ;  a,  large  colos- 
trum-corpuscle ;  b,  small  amoeboid  cells 
containing  oil ;  c,  colostrum-corpuscles 
with  few  oil-droplets. 


suspended.     These  globules  do  not  coalesce,  owing  to  the  probable 
presence  of  a  delicate  envelope  of  casein.     The  addition  of  acetic 

acid  or  of  caustic  potash  destroys  the 
envelope  and  liberates  the  oil-droplets, 
which  then  run  together,  forming  ir- 
regular masses.  Human  milk  is  usu- 
ally alkaline. 

The  milk  secreted  during  the  first 
few  days  after  delivery  contains 
large  fatty  granular  -  looking  bodies 
known  as  colostrum  -  corpuscles  ; 
these  bodies  probably  represent  the  re- 
mains of  a  portion  of  the  epithelial  cells 
which  at  one  time  occupied  the  centre 
of  the  then  solid  acini,  but  which  underwent  fatty  degeneration  and 
partial  destruction  on  the  establishment  of  lactation. 

The  development  of  the  reproductive  organs  comprises  the 
genesis  of  two  distinct  parts,  the  sexual  glands  and  their  excre- 
tory ducts. 

In  order  to  understand  the  formation  of  the  reproductive  organs 
it  is  necessary  to  recall  the  condition  of  the  foetal  excretory  structures 
prior  to  the  appearance  of  the  sexual  glands,  since  the  Wolffian  body 
and  its  duct  play  important  roles  in  the  subsequent  development  of 
the  generative  tract.  The  Wolffian  body  consists  essentially  of  a 
long  tube,  the  Wolffian  duct,  which  extends  parallel  with  the  ver- 
tebral axis  throughout  the  lower  part  of  the  body-cavity,  and  of  the 

transverse      Wolffian 
Fig.  279.  tubules,  which  join  the 

duct  generally  at  right 
angles,  so  that  the  two 
parts  of  the  Wolffian 
body  are  frequently 
compared  to  the  back 
and  the  teeth  of  a  comb. 
The  tubules  are  tor- 
tuous, and  bear  close 
relations  with  tufts  of 
convoluted  capillary 
blood-vessels,  much  the 
same  as  the  uriniferous 
tubules  do  in  the  Mal- 
pighian  bodies  of  the 
kidney.  Some  time  after  the  establishment  of  the  Wolffian  body  and 
its  duct,  a  second  canal,  the  Mullerian  duct,  makes  its  appearance  ; 


Section  of  rabbit  embryo  of  ten  and  a  half  days,  showing  the 
Wolffian  bodies  and  the  early  indifferent  sexual  glands;  tv, t, 
and  »i,  respectively  duct,  tubules,  and  Malpighian  corpuscle 
of  Wolffian  body;  p,  mesothelial  surface  of  primary  peritoneal 
cavity  ;  g ,  indifferent  sexual  glands. 


gep® 


THE    FEMALE    REPRODUCTIVE    ORGAN'S.  2AX 

this  tube  lies  parallel  and  in  close  proximity  with  the  Wolffian  duct,  its 
cephalic  end  opening  into  the  body-cavity,  while  its  lower  extremity 
terminates  at  first  within  the  cloaca  and  later  within  the  uro-genital 
sinus.  It  is  necessary  further  to  distinguish  three  groups  of  the 
Wolffian  tubules,  since  the  fate  of  these  portions  of  the  foetal  oro-an 
varies  ;  these  divisions  are  the  anterior  group,  constituting  the  pro- 
nephros, the  middle  or  sexual  segment,  and  the  posterior  rudi- 
mentary tubules,  which  give  rise  to  atrophic  structures. 

The  development  of  the   sexual   glands    includes  a  primary 
indifferent  and  a  later  specialized  stage. 
During  the  period  when  the  Wolffian  body  Fig.  2S0. 

has  attained  its  greatest  growth   there  ap-       ^L":r.  jfjSi^S^gr^^e 
pears  on  the  ventro-mesial   surface  of  the      '%    -,    ~ 
organ  a  localized  thickening  of  the  meso-       sl^-f'^Wi?^'^ 
thelial  elements.     This  proliferation  pro-       ,c^§P^WIL., 
duces  an  eminence,  the  earliest  trace  of  the         "q^^^t'J:  ~r:  :'f 
sexual  gland.       This  for  some  time   is  in-        Section  of  peripheral  zone  of  in- 
different,    Since    itS    appearance    is    identical       different  sexual  gland  from  rabbit 
11  embryo  :  e,  mesothelial  cells  con- 

in    the    tWO     Sexes.        The    indifferent    SeX-        stituting  the  later  germinal  epitbe- 

ual    glands    soon   exhibit   two   kinds   of     'ium;  s\s'smM  elfements ^ved 

°  from  prohteration  of  mesothelmm  ; 

elements,    the   loosely-packed    proliferated     o,  large  primordial  sexual  ceils, 
small    mesothelial    elements    and    the 
sparingly-distributed  much  larger  primordial  sexual  cells. 

In  the  male,  the  further  changes  within  the  sexual  gland  include 
extended  proliferation  of  the  early  mesothelial  elements  and  their 
grouping  as  epithelioid  cylindrical  masses,  the  sexual  cords  ;  within 
the  latter  lie  the  large  primordial  sexual  cells,  their  number,  however, 
in  the  developing  testicle  being  distinctly  smaller  than  in  the  corre- 
sponding female  organ.  The  sexual  cords  become  the  seminiferous 
tubules,  remaining  solid  cylinders  throughout  foetal  life.  The  par- 
ticular fate  of  the  large  sexual  cells  is  still  uncertain.  The  surround- 
ing mesodermic  tissue  grows  into  the  mesothelial  mass  and  contributes 
the  intertubular  connective  tissue  as  well  as  the  denser  portions  of  the 
framework  represented  by  the  tunica  albuginea  and  the  trabecular. 

The  system  of  canals  forming  the  connection  between  the  testicle 
and  the  epididymis,  including  the  vasa  efferentia  and  the  coni 
vasculosi,  are  derived  from  the  tubules  of  the  Wolffian  body ; 
by  the  ingrowth  of  these  canals  into  the  embryonal  testicle  the  isolated 
sexual  gland  is  provided  with  excretory  passages.  Other  remains 
of  the  lower  tubules  of  the  Wolffian  body  constitute  the  para- 
didymis. The  main  tube  of  the  epididymis  and  the  vas  deferens 
are  the  direct  representatives  of  the  Wolffian  duct. 

The  Miillerian  duct  in  the  male  is  atrophic,  since  its  extreme 
anterior  and  posterior  parts  alone  persist ;  these  remain  as  the  non- 


244 


NORMAL   HISTOLOGY. 


stalked  or  sessile  hydatid,  in  close  relation  with  the  epididymis, 
and  as  the  short  diverticulum  opening  into  the  prostatic  portion  of 
the  urethra,  the  uterus  masculinus  or  sinus  pocularis,  which  is 
therefore  the  homologue  of  the  uterus. 

In  the  female  the  indifferent  sexual  gland  early  exhibits  a  grouping 
of  the  mesothelial  elements  into  cylindrical  masses,  the  sexual  cords, 
which  in  the  ovary,  however,  retain  a  closer  connection  with  the  ger- 
minal mesothelium  than  do  those  of  the  testicle.  Many  groups  of 
epithelial  elements  are  disposed  vertically  to  the  free  surface  of  the 
organ,  and  constitute  the  primary  egg-tubes.  In  the  ovary,  as  in 
the  testicle,  the  sexual  cords  contain  the  large  sexual  cells,  the  latter 
being  much  more  plentiful  in  the  female  than  in  the  male  gland.     The 

ovarian  stroma  originates 
later  as  a  secondary  ingrowth 
of  the  surrounding  mesoderm 
between  the  groups  of  sexual 
cells.  The  genetic  relations 
between  the  embryonal  ele- 
ments, particularly  the  large 
cells,  and  the  ova  and  the 
follicular  cells  of  the  fully- 
formed  ovary,  are  still  indefi- 
nite. It  may  be  assumed  as 
established,  however,  that 
both  these  constituents  of  the 
later  organ  are  derived  from 
the  cells  constituting  the  sex- 
ual cords,  and,  therefore,  in- 
directly from  the  ovarian 
mesothelium  or  primitive 
germinal  epithelium. 
Whether  the  large  sexual  cells  are  the  direct  ancestors  of  the  ova 
alone  or  contribute  to  the  production  of  the  follicular  elements  as 
well  is  uncertain,  but  it  seems  probable  that  the  later  ova  are  the 
immediate  descendants  especially  of  the  large  sexual  cells. 

The  passages  providing  for  the  escape  of  the  product  of  the  ovaries, 
the  ova,  are  derived  from  the  Mullerian  ducts,  their  anterior  seg- 
ments remaining  distinct  tubes,  the  oviducts,  while  their  middle  and 
posterior  divisions  become  fused  and  form  respectively  the  uterus 
and  the  vagina.  The  Wolffian  body  and  its  duct  in  the  female 
are  represented  by  rudimentary  structures,  the  parovarium  and  the 
paroophoron.  The  horizontal  head-tube  of  the  former  is  the 
persistent  anterior  segment  of  the  Wolffian  duct,  while  the  shorter 
vertical  branches  are  the  remains  of  the  "Wolffian  tubules.     The 


Section  of  ovary  from  very  young  kitten  :  a,  ovarian 
mesothelium  or  germinal  epithelium,  containing  large 
sexual  cell  (c) ;  b,  cylindrical  epithelial  masses  consti- 
tuting egg-tubes  ;  d,  developing  stroma. 


THE    FEMALE    REPRODUCTIVE    ORGANS.  2A~ 

presence  of  a  number  of  the  rudimentary  canals  which  constitute  the 
lower  atrophic  segment  of  the  Wolffian  body  produces  the  obscure 


c 


Diagrams  illustrating  development  of  sexual  organs.  In  all  figures  IV,  M,  B,  and  G  represent 
respectively  Wolffian  duct,  Miillerian  duct,  bladder,  and  gut.  A ,  indifferent  type  containing  funda- 
mental parts:  s,  sexual  gland;  t,  t' ,  t" ,  Wolffian  tubules  constituting  anterior  (pronephros),  middle 
(sexual),  and  posterior  (rudimentary)  groups ;  those  of  sexual  division  retain  their  communication 
with  Wolffian  duct.  B,  male  type:  T,  testicle;  e,  e' ,  tubes  of  globus  major  derived  from  middle 
Wolffian  tubules;  v,  tube  of  epididymis,  the  persistent  Wolffian  duct;  s,  seminal  vesicle;  /,  para- 
didymis ;  h' ,  unstalked  hydatid ;  u' ,  uterus  masculinus,  the  persistent  parts  of  the  Miillerian  duct 
(M);  h,  stalked  hydatid;  g,  Cowper's  glands;  m,  penis;  K,  kidney.  C,  female  type:  O,  ovary; 
P,  parovarium;  p' ,  paroophoron;  W,  Gartner's  duct  when  present;/",  fimbria;  o,  oviduct;  u, 
uterus  ;  v,  vagina  ;  h,  stalked  hydatid  ;  K,  kidney.     (Modified  after  Wiedersheim.) 

paroophoron,  the  homologue  of  the  paradidymis.  The  greater 
part  of  the  Wolffian  duct  atrophies  in  the  female  ;  when  it  persists 
as  a  pervious  canal  it  becomes  the  duct  of  Gartner. 


Male. 

Testicle. 

Tubules  composing  glo- 
bus major. 

Paradidymis. 

Tube  of  epididymis  and 
vas  deferens. 

Stalked  hydatid. 

Sessile  hydatid  represent- 
ing fimbria. 

Uterus  masculinus. 

Usually  undeveloped. 

Bladder  and  first  part  of 
urethra. 

Remaining  parts  of  ure- 
thra. 

Cowper's  glands. 

Penis. 

Scrotum. 


Indifferent  Type. 
Sexual  gland. 
Wolffian  tubules. 


Wolffian  duct. 


Duct  of  pronephros. 
Miillerian  duct. 


Lower  segment  of  allan- 

tois. 
Uro-genital  sinus. 


Genital  eminence. 
Genital  ridges. 


Female. 

Ovary. 

Short  tubules  of  parova- 
rium. 

Paroophoron. 

Head-tube  of  parovarium. 
Gartner's  duct  when 
persistent. 

Stalked  hydatid. 

Oviduct. 

Uterus. 
Vagina. 
Bladder  and  urethra. 

Vestibule. 

Bartholin's  glands. 

Clitoris. 

Labia  majora. 


246 


NORMAL    HISTOLOGY. 


CHAPTER   XIV. 

THE    RESPIRATORY    ORGANS. 

The  respiratory  tract  consists  of  two  parts, — the  system  of  air- 
passages,  including  the  nasal  fossae,  pharynx,  larynx,  trachea,  and 
bronchial  tubes,  and  special  organs,  the  lungs,  devoted  to  the  per- 
formance of  the  respiratory  function. 

THE    LARYNX. 

The  larynx  consists  of  the  cartilaginous  framework  formed  by 
the  thyroid,  the  cricoid,  the  arytenoid,  and  the  other  smaller  car- 
tilages of  Wrisberg  and  of  Santorini,  united  by  the  ligamentous 
membranes  and  the  bands  of  fibrous  tissue,  and  lined  within  by 
mucous  membranes  ;  on  the  outside  the  cartilages  are  covered  by 
fibrous  and  muscular  structures. 

The  mucous  membrane  of  the  larynx  possesses  the  same  con- 
stituents as  does  that  of  the  pharynx, — namely,  an  epithelium,  a  tunica 
propria,  and  a  submucosa. 

The  epithelium  covering  both  surfaces  of  the  epiglottis  and  the 
cavity  of  the  larynx  as  far  as  the  false  vocal  cords  is  stratified 
squamous  in  character  ;  at  the  lower  edge  of  the  false  vocal  mem- 
branes the  epithelium  becomes  stratified  ciliated  columnar,  which 
type  is  retained  throughout  the  ventricle  of  the  larynx.  Over  the 
true  vocal  cords  the  epithelium  once  more  becomes  stratified 
squamous,  beyond  which  point  the  stratified  ciliated  columnar 
character  is  again  resumed  and  retained  throughout  the  trachea 
and  the  bronchi.  Numerous  taste-buds,  identical  in  structure  with 
those  of  the  tongue,  lie  embedded  on  the  posterior  surface  of  the 
epiglottis. 

The  tunica  propria  of  the  larynx  is  composed  of  fibrous  connec- 
tive tissue,  with  which  is  mingled  an  especially  rich  net-work  of  elastic 
fibres  ;  in  the  true  vocal  cords,  almost  the  entire  membrane  con- 
sists of  longitudinal  bundles  of  elastic  tissue  ;  these  cords,  therefore, 
are  folds  of  the  mucosa,  composed  principally  of  elastic  fibres,  with 
some  fibrous  tissue,  covered  by  stratified  squamous  epithelium  and 
re-enforced  externally  by  the  fasciculi  of  the  thyro-arytenoideus  mus- 
cle. In  addition,  large  numbers  of  leucocytes  lie  scattered  through- 
out the  mucosa  ;  in  the  posterior  surface  of  the  epiglottis,  the  false 
cords,  and  the  ventricle  of  the  larynx  the  leucocytes  are  so  numerous 


THE    RESPIRATORY    ORGANS.  247 

that  the  mucosa  assumes  the  character  of  diffuse  adenoid  tissue. 
The  superficial  part  of  the  tunica  propria  is  beset  with  papillae,  best 
developed  in  those  regions  which  are  covered  with  squamous  epi- 

Fig.  283. 


m 
I 


Longitudinal  section  of  larynx  of  child,  exhibiting  vocal  cords  and  ventricle  :  a,  surface  above  false 
cord  (b)  covered  with  squamous  epithelium  ;  c,  true  cord  covered  with  squamous  epithelium ;  V,  ven- 
tricle lined  with  ciliated  columnar  epithelium;  d,  ducts  of  mucous  glands  (g)  cut  in  various  direc- 
tions ;  m,  fibres  of  th3-ro-arytenoideus  muscle. 


thelium.  The  deeper  layer  of  the  mucous  membrane  is  of  loose 
structure,  and  passes  into  the  still  looser  tissue  of  the  submucosa, 
which  serves  to  attach  the  mucous  membrane  with  the  surrounding 
firmer  structures.  In  places  the  submucous  tissue  contains  mucous 
follicles,  .2-1  mm.  in  length,  lined  with  columnar  cells,  many  of 
which  are  distended  with  mucous  secretion,  even  to  the  condition  of 


g  NORMAL    HISTOLOGY. 

goblet-cells.     The  minute  groups  of  glands  in  the  epiglottis  lie  em- 
bedded within  the  pits  and  openings  in  its  plate  of  cartilage.     The 

true  vocal   cords   are  destitute  of 
Fig.  284.  mucous  glands. 

The  cartilaginous  frame- 
work of  the  larynx  consists  prin- 
cipally of  hyaline  cartilage ;  to 
this  variety  belong  the  thyroid, 
the  cricoid,  and  the  greater  part 
of  the  arytenoid  cartilages. 
The  epiglottis,  the  apex  and 
the  processus  vocales  of  the  aryt- 
enoid cartilages,  together  with 
the  cartilages  of  Wrisberg  and 
of  Santorini,  are  formed  of  the 
yellow  elastic  variety.  The 
little  nodules  embedded  within  the 
lateral  thyro-hyoid  ligaments,  the 
cartilagines  triticeae,  are  some- 
times composed  of  fibrous,  at  other 
times  of  yellow  elastic  cartilage. 
On  the  outer  side  fibrous  connec- 
tive tissue  connects  the  peri- 
chondrium with  the  surrounding 
structures,  the  attachment  of  the  muscles  being  effected  by 
tendinous  tissue  directly  continuous  with  the  investment  of  the 
cartilage. 

The  blood-vessels  supplying  the  interior  of  the  larynx  terminate 
within  the  mucosa  in  capillary  net-works  beneath  the  epithelium  ;  in 
those  parts  where  papillae  exist  these  are  provided  with  vascular 
loops. 

The  lymphatics  exist  as  a  superficial  net-work  of  small  vessels 
within  the  mucosa,  and  a  deeper  set,  composed  of  much  larger 
channels,  within  the  submucous  tissue  ;  these  latter  vessels  are  of 
exceptional  size  on  the  anterior  surface  of  the  epiglottis.  The  lym- 
phoid character  of  the  mucosa  in  certain  localities  has  already 
been  noted ;  local  aggregations  of  such  cells  in  the  form  of  lymph- 
follicles  are  encountered  in  man  sometimes,  and  constantly  in  some 
of  the  lower  animals  (dog,  cat). 

The  nerves  distributed  to  the  laryngeal  mucous  membrane  are 
composed  principally  of  medullated  fibres,  although  pale  fibres  are 
present. 


Longitudinal  section  of  epiglottis  of  child : 
a,  laryngeal  surface  ;  b,  glossal  surface  ;  c,  plate 
of  elastic  cartilage  ;  d,  acini  of  mucous  glands. 


THE   RESPIRATORY   ORGANS, 


249 


trachea  is  lined  by  stratified 


Fig 


THE   TRACHEA. 

The  trachea  in  its  general  structure  resembles  the  lower  part  of  the 
larynx  :  it  consists  of  a  fibrous  tube,  lined  by  the  mucous  mem- 
brane, and  strengthened  and  kept  open  by  a  series  of  incomplete 
cartilaginous  rings. 

The  mucous  membrane  of  the 
ciliated  columnar  epithelium, 
among  whose  elements  lie  num- 
bers of  goblet-cells.  The  current 
established  by  the  ciliated  epithe- 
lium tends  to  expel  mucus  or  other 
substances. 

The  tunica  propria  is  conspic- 
uous on  account  of  the  large 
amount  of  elastic  tissue  which 
it  contains  ;  owing  to  the  disposi- 
tion of  the  elastic  fibres,  two  zones 
are  recognizable,  an  inner  loosely- 
thrown-together  fibrous  layer, 
containing  some  elastic  tissue,  vas- 
cular loops,  and  nerve-fibres,  to- 
gether with  numerous  lymphoid 
cells,  and  an  outer  layer,  next  the 
submucosa,  made  up  largely  of 
close  net-works  of  longitudinal 
elastic  fibres.  The  elastic  fibres 
are  particularly  robust  and  abun- 
dant along  the  posterior  membra- 
nous wall  of  the  trachea,  between 
the  ends  of  the  cartilages. 

The  submucosa  is  loosely  ar- 
ranged, and  connects  the  mucosa  with  the  fibrous  sheath,  as  well 
as  supports  the  glands  and  larger  blood-vessels,  lymphatics,  and 
nerve-trunks.  The  tracheal  glands  are  represented  by  numerous 
small  groups  of  racemose  structures  which  occupy  the  submucous 
layer  and  communicate  with  the  mucous  surface  by  means  of  the 
long  excretory  ducts.  The  latter  are  lined  with  low  columnar 
epithelium,  while  the  acini  contain  cuboidal  cells. 

The  fibrous  coat  lies  external  to  the  submucosa  and  forms  a 
complete  investment  in  which  the  cartilaginous  rings  are  embedded. 
These  latter  are  C-shaped  masses  of  hyaline  cartilage,  embracing 
almost  three-fourths  of  the  tracheal  tube.  The  remaining  cleft  is 
bridged  by  the  continuation  of  the  fibrous  tunic  supplemented  by 


Section  embracing  trachea  and  oesophagus  of 
child  :  a,  b,  tracheal  and  oesophageal  surfaces  ; 
c,  tracheal  epithelium  ;  d,  stroma  of  mucosa ;  e, 
submucosa;^  mucous  glands;  h,  part  of  ring 
cartilage ;  g,  its  perichondrium ;  i,  fibrous 
tissue  ;  k,  fibro-muscular  tissue  of  oesophagus  ; 
/,  oesophageal  epithelium. 


2r0  NORMAL    HISTOLOGY. 

a  layer  of  transversely-disposed  bundles  of  non-striped  muscle. 
These  latter  extend  for  some  little  distance  along  the  inner  side  of  the 
cartilages,  to  whose  perichondrium  they  are  attached.  The  muscle 
not  only  exists  between  the  ends  of  the  cartilaginous  plates,  but  also 
passes  across  in  the  intervals  between  these,  thus  constituting  a  con- 
tinuous layer,  which  serves  to  narrow  the  tube.  In  addition  to 
the  transverse  bundles,  a  few  longitudinal  muscular  bands  are 
present.  The  outer  surface  of  the  fibrous  tunic  is  connected  with 
the  surrounding  structures  by  loose  areolar  tissue. 

The  larger  blood-vessels  pass  to  the  submucosa,  from  which 
smaller  twigs  are  given  off  to  supply  the  mucous  membrane  and, 
partially,  the  fibrous  and  cartilaginous  structures.  The  vessels  termi- 
nate within  the  mucosa  in  a  net-work  beneath  the  epithelium ;  the 
acini  of  the  mucous  glands  within  the  submucous  layer  are  surrounded 
by  capillaries. 

The  lymphatics  of  the  trachea  are  numerous  within  the  mucous 
and  submucous  coats,  where  they  constitute  plexiform  arrangements 
of  large,  irregular,  thin-walled  channels.  Lymphatic  tissue  in  the 
form  of  solitary  follicles  also  occurs. 

The  nerves  contain  both  medullated  and  non-medullated  fibres. 
The  larger  trunks  pass  within  the  submucosa  and  send  smaller  fibres 
into  the  mucosa,  where  they  course  as  minute  naked  fibrillar  ;  the 
exact  mode  of  their  ending  is  unknown. 

THE    BRONCHI. 

The  larger  bronchial  tubes  repeat  almost  exactly  the  structure 
of  the  trachea,  with  such  modifications  as  result  from  the  slighter 
general  development  of  the  several  coats  incidental  to  the  gradual 
reduction  in  the  size  of  the  tubes. 

On  reaching  the  small  bronchi  the  epithelium  is  reduced  to 
a  single  layer  of  ciliated  columnar  cells.  The  thickness  of  the 
mucosa  at  first  is  not  greatly  diminished,  since  the  loss  sustained  in 
the  thinning  of  the  elastic  tissue  of  the  tunica  propria  is  compensated 
by  the  appearance  of  an  additional  layer  of  non-striped  muscle 
situated  at  the  outer  border  of  the  mucosa,  next  the  submucosa  ;  this 
layer,  which  corresponds  to  a  muscularis  mucosae,  forms  a  com- 
plete investment,  especially  conspicuous  when  the  cartilaginous  plates 
diminish.  The  ring-cartilages  of  the  bronchi  become  reduced  in 
size,  then  broken  up,  and  finally  replaced  by  irregular  short  plates  ; 
these,  becoming  smaller  and  infrequent,  embrace  gradually  less  of 
the  circumference  of  the  tube,  until  in  the  bronchial  twigs  of  the 
diameter  of  about  one  millimetre  they  altogether  disappear. 

By  repeated  division  the  bronchial  tubes  become  greatly  reduced 
in  size,   the  reduction   being  accompanied  by  the  changes  already 


THE    RESPIRATORY    ORGANS. 


251 


noted  ;  when  the  diameter  of  the  twig  no  longer  exceeds  one  milli- 
metre the  tube  is  termed 

a  terminal  bronchus  ;  Fig.  286. 

these  divisions  open  into     ^'-^^j^, 

the  somewhat  larger  al-     t^-jAh;';, '/.-•  '»**, ~~  '  -       . .,,^  \-  ^""^fv 

veolar  passages,  the  "^'*  ~~  rr -.■"""----—"' :r~ 
walls  of  which  are  beset  ^^.^V17^^/;^T^wi;?>i;*'-^;^  ' c-'W^'-. 
with  air-sacs,  and  from 
which  extend  blind  ir- 
regular or  pyramidal 
spaces,  the  infundi- 
bula ;  each  infundibu- 
lum  is  surrounded  on  all 
sides  by  the  air-sacs, 
which  communicate 
freely  with  the  former 
cavity,  but  not  directly 
with  each  other.  Greater 
exactness  suggests  ad- 
ditional subdivisions  of 
the  alveolar  duct  into 
vestibule,  atrium,  and  infundibular  passage    Miller). 

The  walls  of  the  terminal  bronchial  tubes  consist  at  hrst  of  a 
single  layer  of  ciliated  columnar  epithelium, 
outside  of  which  the  mucosa  contains  longitudinal 
elastic  fibres  and  thin,  irregular,  annular  bun- 
dles of  non-striped  muscle.  The  mucous 
glands  and  the  cartilaginous  plates  are  want- 
ing in  the  terminal  tubes.  Within  the  latter 
the  ciliated  cells  disappear,  a  simple  columnar 
epithelium  existing  for  some  distance,  which,  in 
turn,  is  replaced  bv  cuboidal  cells  on  approach- 
ing the  alveolar  ducts. 

The  walls  of  the  alveolar  ducts  suffer  still 
further  reduction,  the  fibrous  coat  becoming 
greatly  thinned,  while  the  mucosa  is  reduced  to 
a  delicate  tunica  propria  of  fibro-elastic  tissue,  in 
which  bundles  of  non-striped  muscle  remain. 

The  epithelium  of  the  alveolar  passage,  at 
first  low  cuboidal  in  character,  rapidly  assumes  a 
flat  polygonal  tvpe :  towards  the  infundibula 
large  flat  plates  appear  among  the  smaller 
polvgonal  cells,  and  become  more  numerous  as  these  terminal 
divisions  of  the  air-passages  are  neared. 


Section  of  portion  of  bronchus  of  child:  a,  epithelium;  b, 
basement-membrane ;  c,  stroma  of  mucosa ;  d,  layer  of  in- 
voluntary muscle ;  e,  submucosa  ;  f,  acini  of  mucous  glands  ; 
h,  blood-vessels ;  i,  obliquely-cut  duct  of  mucous  glands. 


Fig.  2S7. 


Diagram  of  terminal 
compartments  of  air- 
passage :  T.B.,  terminal 
bronchus  ;  A  .D.,  alveo- 
lar ducts  ;  Inf.,  infundi- 
bula, into  -which  open 
air-sacs.  The  general 
character  of  the  epithe- 
lial linin?  is  indicated. 


2c2  NORMAL   HISTOLOGY. 

Within  the  infundibulum  the  epithelial  lining  consists  principally 
of  the  large  flat  endothelioid  plates,  or  respiratory  epithelium,  be- 
tween which  elements  diminutive  groups  of  the  smaller  polygonal 
cells  appear.  In  the  air-sacs,  presently  to  be  described,  the  large 
plate-like  elements  of  the  respiratory  epithelium  chiefly  constitute 
the  lining. 

THE    LUNGS. 

The  lungs,  with  their  system  of  air-tubes,  correspond  in  plan  of 
structure  and  in  development  to  racemose  glands,  the  excretory 
ducts  being  represented  by  the  bronchial  tubes,  and  the  glandular 
tissue  by  the  pulmonary  parenchyma.  The  latter  is  built  up  of 
groups  of  air-sacs  enclosed  by  connective  tissue  to  form  lobules, 
which  are  associated  in  larger  groups  ;  these  latter  in  turn  are  united 
into  the  lobes.  All  these  divisions  are  connected  by  alveolar 
tissue,  the  external  surface  being  additionally  covered  by  the 
pleura. 

By  the  division  of  the  terminal  bronchial  tube  into  the  alveolar 
ducts,  and  the  subsequent  origin  from  these  of  the  infundibula  and 
the  air-sacs,  the  part  of  the  pulmonary  parenchyma  in  communica- 
tion with  a  single  terminal  bronchiole  forms  a  pyramidal  mass, 
whose  apex  corresponds  to  the  terminal  bronchus,  and  whose 
base,  when  reaching  the  free  surface,  appears  as  one  of  the  polygo- 
nal areas  marking 
the  exterior  of  the 
lung.  These  larger 
polygonal  fields, 
made  up  of  many 
smaller  areas  which 
correspond  to  the 
compressed  infundi- 
buli,  are  often  defined 
with  great  distinct- 
ness by  the  pigment 
accumulated  within 
the  connective  tissue 
separating  the  adja- 
cent lobules. 

The  air-sacs,  air- 
cells,  or  alveoli  of 
the    lung    represent 
the  acini  of  racemose 
glands,   the    similarity  being   especially  marked   in   the   uninflated 
organ,  which  still  retains  its  glandular  character.     Opening  into  the 
common  passages  of  the  alveolar  ducts  and  the  infundibula,  the  air- 


Section  of  human  lung  :  a,  infundibula  cut  in  various  directions  ; 
b,  air-sacs  separated  by  interacinous  partitions  (c) ;  d,  masses  of 
interlobular  tissue  containing  accumulations  of  pigmented  par- 
ticles (if). 


THE    RESPIRATORY   ORGANS. 


253 


sacs  are  placed  closely  side  by  side,  and  by  mutual  pressure  become 
polyhedral.  Around  the  opening  or  base  of  the  air-sac,  where 
it  communicates  with  the  infundibulum,  the  elastic  tissue  of  the 
latter  is  arranged  as  a  ring,  from  which  elastic  fibres  pass  in  all 
directions  over  the  air-sac  to  form  its  framework. 

The  wall  of  the  air-sac  comprises  the  epithelium,  the  con- 
nective-tissue framework,  and  the  capillary  net-work. 

The  epithelial  lining  is  represented  chiefly  by  a  single  layer 
of  large  plates,  closely  resembling  endothelium  in  silvered  prepara- 
tions,  among  which 

small     polyhedral  Fig.  289. 

cells  lie  scattered  as 
isolated  elements  or 
in  groups  of  two  or 
three.  Originally, 
in  the  embryonal 
condition  of  the  tis- 
sue, only  the  smaller 
polyhedral  cells  are 
present  in  the  air- 
sacs  and  the  infun- 
dibulum, the  large 
plate-like  elements 
first  appearing  after 
the  tissue  has  been 
expanded  following 
inflation  of  the 
organ.       The   small 

cells,  therefore,  are  to  be  regarded  as  genetically  identical  with 
the  larger,  the  only  difference  being  that  the  smaller  have  never 
undergone  the  expansion  to  which  their  neighbors  have  been  sub- 
jected ;  during  forced  expiration  the  larger  cells  become  diminished 
in  size.  Between  the  cells,  frequently  at  the  juncture  of  the  angles 
of  several,  minute  openings  or  stomata  exist ;  they  usually  connect 
with  microscopic  passages  leading  into  the  lymphatic  channels.  By 
means  of  these  channels  particles  of  inhaled  foreign  matters, 
often  deeply  pigmented,  are  carried  from  the  air-sacs  into  the  lym- 
phatics, and  become  lodged  within  the  interlobular  connective  tis- 
sue. Additional  particles  are  carried  into  the  tissues  by  means  of 
the  wandering  lymphoid  cells  which  occur  within  the  epithelium 
of  the  air-sacs  and  air-passages. 

The  framework  of  the  air-sac  is  composed  almost  entirely  of  the 
elastic  fibres  springing  from  the  annular  bundle  surrounding  the 
mouth  of  the  sac.      These  fibres  unite  to  form  a  net-work  which 


Section  of  silvered  lung  of  kitten,  including  portions  of  infun- 
dibulum and  air-sac :  a,  small  polyhedral  epithelial  cells  covering 
wall  of  infundibulum  ;  b,  nbro-elastic  framework  ;  c,  large  flattened 
epithelial  plates  lining  air-sac,  among  which  lie  small  groups  of  the 
small  cells  (d). 


254 


NORMAL    HISTOLOGY. 


Fig 


completely  surrounds  the  alveolus  and  constitutes  the  septum  be- 
tween the  adjoining  air-sacs,  at  the  same  time  supporting  the  capillary 
vessels  and  the  investing  epithelium.  In  addition  to  the  elastic  fibres, 
a  very  small  quantity  of  fibrous  tissue,  with  a  few  connective-tissue 
cells,  aids  in  the  construction  of  the  air-sac. 

The  capillary  net-work  within  the  walls  of  the  air-sacs  is  re- 
markable for  the  closeness  of  its  meshes,  being  one  of  the  densest 
vascular  net-works  within  the  body.     The  larger  arterial  stems 

take  theircourse,  in  company  with 
the  veins,  bronchioles,  nerves,  and 
lymphatics,  within  the  thicker 
tracts  of  interlobular  connec- 
tive tissue  ;  the  smaller  twigs 
extend  among  the  groups  of  in- 
fundibula,  embracing  the  openings 
into  the  air-sacs  with  more  or  less 
complete  rings,  from  which  pass 
the  capillaries  enveloping  the  air- 
sacs  with  net-works  on  all  sides. 
Between  the  adjoining  alveoli  lies 
only  a  single  layer  of  capillary 
vessels,  which,  however,  are  not 
confined  to  a  single  plane,  but 
encroach  alternately  upon  the 
adjacent  air-sacs  as  projecting  arches  or  loops. 

While  the  interalveolar  septa  are  reduced  to  a  minimum,  the  two 
layers  of  respiratory  epithelium  lining  the  adjoining  air-sacs,  the 
scanty  connective-tissue  framework  and  the  capillary  net- 
work constituting  their  entire  bulk,  the  alveoli  belonging  to  differ- 
ent, although  neighboring,  infundibula  are  separated  by  distinct 
connective-tissue  partitions ;  these  increase  in  thickness  as  the 
included  divisions  of  pulmonary  substance  become  larger,  and  reach 
their  greatest  development  in  the  fibrous  envelopes  ensheathing 
and  separating  the  lobes. 

Owing  to  the  accumulation  of  the  pigmented  particles  conveyed 
by  the  lymphatics  in  the  manner  already  described,  the  interinfun- 
dibular  and  often  also  the  interlobular  connective  tissue  present  dark 
patches,  the  degree  of  discoloration  varying  from  a  few  scattered 
irregular  points  to  an  intense  almost  uniformly  black  area.  The 
presence  of  pigment  within  the  connective  tissue  emphasizes  the 
outlines  and  boundaries  of  the  lobules  with  diagrammatic 
sharpness. 

The  blood-vessels  of  the  lungs  enter  at  the  hilus  along  with  the 
large  divisions  of  the  bronchus  ;  the  smaller  branches  of  the  pulmo- 


Section  of  injected  and  inflated  lung  of  cat : 
a,  air-sacs  enclosed  in  dense  capillary  net-works 
(b)  ;  c,  larger  interlobular  branches  of  pulmonary 
artery. 


THE   RESPIRATORY   ORGANS.  2  " 

nary  artery  follow  the  air-tubes  to  their  ultimate  distribution,  the 
arterioles  extending  along  the  respiratory  bronchial  tubes  and  alveolar 
ducts  as  far  as  the  infundibular  septa.  They  there  end  in  capil- 
lary net-works  which  surround  the  air-sacs  in  the  manner  above 
described.  In  their  course  along  the  respiratory  bronchial  tubes  and 
the  alveolar  ducts  the  pulmonary  arterioles  give  off  twigs  which  form 
net-works  around  the  air-sacs  besetting  those  passages.  The 
blood  of  the  alveolar  net- works  is  carried  away  by  the  radicles  of  the 
pulmonary  veins,  which  begin  at  the  margins  of  the  air-sacs. 

In  addition  to  the  system  of  vessels  derived  from  the  pulmonary 
artery  destined  for  the  respiratory  function,  a  second  group,  for 
the  nutrition  of  the  pulmonary  tissues,  is  distributed  by  the 
bronchial  arteries.  These  vessels  run  in  company  with  the  bron- 
chial tubes  and  the  other  blood-vessels  within  the  interlobular  con- 
nective tissue  and  give  off  twigs  which  break  up  into  the  capillaries 
immediately  supplying  the  walls  of  the  air-passages  and  associated 
structures.  Additional  capillaries  supply  the  interlobular  areolar 
tissue  and  the  pleural  tissues  on  the  surface  of  the  lungs. 

The  numerous  lymphatics  of  the  lung  are  arranged  as  two  sets, 
those  originating  within  the  connective-tissue  septa  and  those 
arising  in  connection  with  the  bronchial  mucous  membranes. 
Of  the  former  two  groups  are  recognized,  one  of  which  includes  the 
channels  beginning  within  the  interlobular  fibrous  tissue  and 
forming  the  lymphatics  which  accompany  the  branches  of  the  pul- 
monary blood-vessels  ;  the  other,  the  superficial  set,  arises  by  the 
radicles  connected  with  the  subpleural  lymph-spaces,  which 
communicate  with  the  serous  cavity  of  the  pleura  by  means  of  the 
minute  passages  leading  from  the  intercellular  orifices  of  the  pleural 
surface  into  the  subjacent  lymph-clefts. 

The  bronchial  lymphatics  originate  within  the  subepithelial 
lymph-spaces  which  communicate  with  the  mucous  surfaces  of  the 
air-tubes  and  the  alveoli  through  the  stomata;  from  the  subepithelial 
plexus  larger  lymph-channels  unite  with  others  to  form  definite  lym- 
phatic canals  ;  these  accompany  the  blood-vessels  to  the  root  of  the 
lung,  where  the  superficial  and  deep  lymphatics  meet  and  are  taken 
up  by  a  few  trunks  of  large  size  which  pass  from  the  lung  to  the 
bronchial  lymph-glands.  Masses  of  lymphoid  tissue  of  varying 
extent  are  associated  with  the  walls  of  the  alveolar  ducts  and  the 
bronchial  tubes,  as  well  as  the  subpleural  and  peribronchial  areolar 
tissue. 

The  nerves  of  the  lung  include  contributions  from  both  the  cerebro- 
spinal and  the  sympathetic  system.  The  nerve-trunks,  made  up 
of  medullated  and  pale  fibres,  enter  the  organ  at  its  root  and  follow 
the  air-tubes  and  the  blood-vessels.     Small  groups  of  ganglion-cells 


2rg  NORMAL   HISTOLOGY. 

occur  along  their  course.  On  reaching  the  smaller  and  the  terminal 
ramifications  of  the  bronchial  tubes  the  nerves  become  broken  into 
fine  non-medullated  fibrillae,  which  pass  to  the  muscular  tissue  of  the 
tubule  as  well  as  to  the  mucous  membrane.  The  exact  mode  of 
final  termination  of  the  nerve-filaments  within  the  pulmonary  tissue 
is  still  undetermined. 

THE    PLEURA. 

The  pleura  resembles  in  structure  other  serous  membranes,  the 
general  characters  of  which  have  been  already  considered  in  Chapter 
VIII.       It  consists  of  an   endothelial   covering,  a  connective- 
tissue  matrix,  and  subpleural  tissue.     The  lining  of  the  pleural 
cavity  is  not  of  equal  thickness  in  all  parts, 
Fig.  291.  the  visceral  or  pulmonary  pleura  being  thin- 

nest as  well  as  most  firmly  attached,  while 
the  parietal  or  costal  pleura  is  thickest,  and, 
owing  to  the  well-developed  subpleural  tissue 
existing  in  this  region,  less  rigidly  adherent. 
The  endothelium  of  the  parietal  portion 
Section  of  human  pleura  cover-     possesses  cens  more  expanded  and  thinner 

ing  surface  of  lung :  a,  endothe-       r  .  .  - 

Hum;  b, fibro-eiastic stroma ;  m,  than  those  covering  the  surface  of  the  lung; 
cut  bundle  of  muscie-ceiis;  p,     tlie  eiements  [n  this  latter  position  vary  in 

peripheral   layer   of   pulmonary         ...  . 

tiSSue.  their  size  with  the  changes  in  the  bulk  of 

the  pulmonary  mass.  Between  the  endo- 
thelial plates  minute  stomata  exist,  which  through  the  minute  cana- 
liculi  indirectly  communicate  with  the  lymphatic  spaces  within  the 
subjacent  tissue. 

The  stroma  of  the  pleura  consists  of  fine  bundles  of  fibrous 
connective  tissue  intermingled  with  elastic  fibres  ;  within  the  fibrous 
lamellae  the  intercommunicating  lymph-channels  form  a  plexus  of 
considerable  richness,  which  communicates  on  the  one  hand  with 
the  pleural  cavity  through  the  stomata  and  intervening  canaliculi,  and 
on  the  other  with  the  neighboring  lymphatics  within  the  subpleural 
tissue. 

The  latter  where  developed  as  a  layer  of  some  thickness,  as  beneath 
the  parietal  pleurae,  is  composed  of  loosely-disposed  areolar  tissue, 
containing  many  elastic  fibres.  Upon  the  lung  the  subpleural  layer 
is  intimately  united  with  the  pulmonary  tissue,  and  forms  a  strong 
superficial  fibrous  envelope,  in  which  bundles  of  non-striped 
muscle  are  also  present.  Within  the  stroma  of  the  visceral  pleura 
the  blood-vessels  form  a  wide-meshed  capillary  reticulum  over  the 
surface  of  the  lung  ;  superficial  vessels  communicate  with  deeper 
branches  surrounding  the  interalveolar  septa. 

The  nerves  of  the  pleura  occur  as  infrequent  stems,  composed 


THE   RESPIRATORY   ORGANS.  257 

principally  of  medullated  fibres  ;  fibrillae  are  traceable  into  the  sub- 
pleural  tissue,  but  their  exact  mode  of  ending  is  uncertain. 

THE   THYROID    BODY. 

In  view  of  its  topographical  relations,  as  well  as  a  matter  of  con- 
venience, it  is  usual  to  consider  this  organ  in  connection  with  the 
respiratory  tract,  although  such  association  is  only  incidental  and 
without  foundation  or  morphological  significance,  unless  its  descent 
in  common  with  the  respiratory  organs  as  an  outgrowth  from  the 
pharyngeal  entoderm  be  regarded  in  such  light. 

The  thyroid  body  is  a  compound  tubular  gland  whose  excre- 
tory canal,  the  thyro-glossal  duct,  in  the  early  stages  of  the  organ, 
connects  the  tubules  with  the  mucous  surface,  where  its  opening 
corresponds  to  the  foramen  caecum,  situated  on  the  dorsum  about 
an  inch  from  the  base  of  the  tongue.  After  a  short  existence,  long 
before  the  gland  attains  its  full  development,  the  thyro-glossal  duct 

Fig.  292. 


Section  of  thyroid  body  of  child :   a,  acini  distended  with  colloid  secretion,  cut  in 
various  directions;  b,  interlobular  connective  tissue. 

undergoes  atrophy  and  more  or  less  complete  obliteration  ;  the  acini, 
consequently,  become  isolated  closed  cavities,  while  the  organ  is 
often  classed  as  a  ductless  gland. 

The  fully-developed  adult  thyroid  gland  consists  of  numerous 
tubular  acini,  40-110  /j.  in  diameter,  united  by  intertubular  areolar 
tissue  into  lobules ;  these,  in  turn,  are  joined  into  lobes  by  still 
larger  masses  of  connective  tissue,  which  form  on  the  outside  of  the 
organ  a  general  external  fibrous  envelope. 

The  acini  are  completely  closed,  and  lined  with  a  single  layer  of 
cuboidal  or  low  columnar  epithelium,  whose  component  cells  rest 
upon  a  distinct  basement-membrane.     The  enclosed  cavities  differ 

17 


Section  of  thyroid  body,  exhibiting  de- 
tail of  the  acini,  which  are  cut  in  various 
directions :  c,  colloid  material  distending 
the  larger  acini ;  i,  interacinous  connective 
tissue ;  v,  blood-vessels. 


2-g  NORMAL   HISTOLOGY. 

according  to  the  size  and  the  distention  of  the  acini ;  they  usually  con- 
tain a  viscid  yellowish  mass,  the  colloid  substance,  produced  through 

the  active  agency  of  the  cells  lining  the 
acini.  In  addition  to  the  characteris- 
tic colloid  secretion,  detached  epithe- 
lium, leucocytes,  migrated  plasma- 
cells,  and  in  very  many  cases  colored 
blood-corpuscles,  are  included  within 
the  contents  of  the  alveoli.  The  pres- 
ence of  red  blood-cells  in  various  stages 
of  disintegration  has  suggested  the 
destruction  of  effete  blood-cells  as  a 
possible  function,  in  part  at  least,  of 
this  questionable  organ.  The  inter- 
alveolar  tissue  contains  elements  closely 
resembling  plasma-cells. 

The  blood-vessels  of  the  thyroid 
gland  are  exceptionally  numerous,  the  arteries  being  remarkable  for 
their  large  size  and  very  free  anastomoses.  From  the  larger 
branches,  which  run  within  the  interlobular  tissue,  small  twigs  pass 
between  the  alveoli  and  break  up  into  capillaries  surrounding  the 
acini  with  a  close-meshed  net-work  situated  immediately  beneath 
their  epithelium.  The  venous  radicles  are  also  numerous,  and  form 
the  conspicuous  superficial  plexuses. 

The  plentiful  lymphatics  occupy  the  deeper  connective-tissue 
septa  between  the  lobules  as  well  as  the  fibrous  envelopes  surrounding 
the  lobes.  The  deeper  lymphatics  begin  as  spaces  lying  between 
the  bundles  of  fibrous  tissue  close  to  the  acini,  and  frequently  contain 
characteristic  colloid  substance.  Large  superficial  trunks,  provided 
with  valves,  carry  off  the  accumulations  from  the  smaller  canals. 

The  few  nerves  which  supply  the  thyroid  gland  are  derived  almost 
entirely  from  the  sympathetic  system.  The  fibres,  therefore,  are  prin- 
cipally of  the  pale,  non-medullated  variety,  and  seem  to  be  distributed 
especially  to  the  walls  of  the  blood-vessels  ;  a  few  medullated  fibres  are 
usually  present,  but  the  exact  mode  of  their  termination  is  uncertain. 
The  development  of  the  respiratory  organs  begins  as  a  ven- 
tral evagination  of  the  entodermic  lining  of  the  primitive  pharynx. 
The  caudal  extremity  of  this  complex  cavity  abruptly  narrows  into 
the  oesophageal  division  of  the  primary  gut  tract.  The  earliest  in- 
dication of  the  formation  of  the  respiratory  apparatus  consists  in  the 
extension  of  the  ventro-dorsal  diameter  of  the  primitive  oesophagus 
at  its  pharyngeal  end,  in  which  plane  it  now  appears  as  an  irregularly- 
compressed  ellipse. 
The  pulmonary  evagination  extends  caudally  for  some  distance, 


Fig 


THE   RESPIRATORY   ORGANS.  2rg 

when  its  expanded  extremity  divides  into  two  lateral  diverticula  ■ 
of  these,  the  right,  which  is  the  larger  and  longer, 
subdivides  into  three  branches,  while  the  left 
bears  but  two.  These  pouches  correspond  to 
the  future  lobes,  and  thus  early  establish  the 
asymmetrical  division  of  the  future  lungs.  The 
pharyngeal  end  of  the  pulmonary  tube  becomes 
the  larynx,  while  the  remaining  portions  form 
the  system  of  air-passages,  including  the  tra- 
chea and  the  bronchial  tree.  In  the  further 
development  of  the  bronchial  ramifications  the 
same  general  plan  of  division  is  repeated.  The 
already-existing  tube  divides  dichotomously, 
but  the  limbs  of  the  forks  grow  unequally,  since 
the  ventral  bud  becomes  the  continuation  of 
the  stem,  while  the  other  becomes  a  lateral 
branch.  After  the  entire  system  of  air-passages 
is  established  the  expanded  ends  of  the  terminal 
buds  produce  the  ultimate  divisions  of  the  pul- 
monary structure.  While  the  entodermal 
diverticulum  thus  takes  part  in  the  formation 
of  the  entire  pulmonary  tract,  its  contribution 
is  limited  to  the  epithelial  lining  of  the  alveoli 
and  the  air-tubes,  while  the  remaining  constituents  of  the  respiratory 
organs  are  derived  from  the  mesoderm.     The  mesodermic  tissue 


Part  of  sagittal  section 
of  eleven-day  rabbit  em- 
bryo, showing  pulmonary 
evagination :  P,  primitive 
pharynx  ;  r,  o,  respiratory 
and  oesophageal  tubes ;  b, 
body-cavity ;  in,  mesoder- 
mic tissue. 


Fig.  295. 


Portion  of  section  of  thirteen-day  rabbit  embryo,  including  developing  lungs  ;  mesodermic  pulmo- 
nary masses,  L,  L! ',  are  covered  with  primary  pleural  endothelium  and  penetrated  by  bifurcations  of 
primary  bronchi  (t,  t) ;  v,  blood-vessels  ;  o,  oesophageal  tube. 


surrounds  the  entodermic  diverticula,  constituting  for  a  time  a  con- 
spicuous mass,  into  which  the  epithelial  tubes  grow.  Subsequently 
the  mesodermic  area  becomes  so  completely  invaded  by  the  rapidly- 


26o 


NORMAL   HISTOLOGY. 


Fig.  296. 


developing  system  of  primary  air-tubes  and  alveoli  that  its  relative 
quantity  is  greatly  reduced,  since  it  eventually  is  limited  to  the 
connective-tissue  framework  of  the  organ. 
The  appearance  of  the  blood-vessels  occurs  at 
a  later  period.  The  derivation  of  the  greater 
part  of  the  digestive  and  of  the  respiratory 
tract  is  identical, — namely,  the  epithelial 
structures  from  the  entoderm  and  the  re- 
maining tissues  from  the  mesoderm. 

The  development  of  the  thyroid  body 
includes  the  history  of  two  structures  which 
originate  independently,  but  which  after  a  short 
time  in  man  and  other  mammals  become 
blended  to  constitute  a  single  organ  :  in  many 
animals,  however,  the  mesial  and  lateral  thy- 
roid areas  produce  organs  which  permanently 
remain  distinct. 

The  middle  thyroid  area,  from  which  originates  the  true  thyroid 
body,  appears  as  a  ventral  outgrowth  from  the  entodermic  lining 
of  the  primitive  pharynx  at  a  position  corresponding  approxi- 
mately with  the  second  visceral  arch.  The  mesial  outgrowth  rapidly 
elongates,   and  after  a  time  usually  loses  its  attachment  with  the 


Portion  of  sagittal  section 
of  twelve-day  rabbit  em- 
bryo, exhibiting  mesial  thy- 
roid area  as  epithelial  out- 
growth (f)  still  connected 
with  pharyngeal  entoderm 
(e) ;  tn,  surrounding  meso- 
derm. 


Fig.  297. 

/' 


pharyngeal  epithelium.  The  entodermic  mass 
gradually  leaves  the  primitive  pharynx  and  as- 
sumes a  close  relation  with  the  paired  lateral 
thyroid  areas,  with  which  it  eventually  fuses. 

The  lateral  developmental  areas  of  the 
thyroid  body  appear  as  ventral  outgrowths 
from  the  entodermic  lining  of  the  fourth  inner 
visceral  furrow  on  either  side.  The  epithelial 
evaginations  become  elongated  cylindrical 
masses,  which  undergo  active  proliferation  and 
extend  their  bulk  as  branching  cords  ;  where 
these  are  at  first  solid  they  subsequently  obtain 
a  lumen,  and  for  a  time  present  the  character  of 
tubular  glands.  The  later  changes  include  the 
approximation  and  final  fusion  of  the  two  lateral 
and  the  single  mesial  areas  to  form  the  thyroid 
body  of  the  mammalian  type.  The  disappear- 
ance of  excretory  ducts  and  the  ingrowth 
of  the  surrounding  mesoderm  result  in  the  division  of  the  organ  into 
lobules  and  the  isolation  of  the  imperfectly-developed  acini.  Disten- 
tion of  the  latter  by  accumulations  of  colloid  material  follows  the 
activity  of  the  secreting  cells  within  the  ductless  alveoli. 


Portion  of  section  of  four- 
teen-day rabbit  embryo,  in- 
cluding lateral  thyroid  area 
(r)  which  is  still  attached 
to  fourth  inner  pharyngeal 
furrow  (/) ;  m,  surrounding 
mesoderm. 


THE   SKIN   AND    ITS   APPENDAGES. 


26l 


CHAPTER    XV. 


THE   SKIN   AND    ITS    APPENDAGES. 


THE    SKIN. 


The  skin  consists  of  two  parts  :  the  superficial  epithelial  layer — 
the  epidermis  or  the  cuticle,  derived  from  the  ectoderm — and  the 
deeper  connective-tissue  layer — the  corium  or  the  cutis  vera,  de- 
rived from  the  mesoderm.  Blended  with  the  corium  and  separated 
from  it  by  no  sharp  demarcation,  the  subcutaneous  tissue  exists 

Fig.  29S. 


>*£ 


Section  of  human  skin  :  a,  stratum  corneum;  b,  stratum  lucidum  ;  c,  stratum 
granulosum  ;  d,  stratum  Malpighii ;  e,  f,  papillary  and  reticular  layers  of  corium  ; 
g,  stratum  of  adipose  tissue  ;  h,  i,  spiral  and  straight  portions  of  duct  of  sweat- 
gland  ;  k,  coiled  portion  of  sweat-gland ;  I,  vascular  loops  occupying  papillae  of 
corium. 

usually  as  a  stratum  of  considerable  thickness,  which  forms  a  loose 
attachment  between  the  skin  and  the  adjacent  structures.  The  in- 
tegument varies  in  thickness  from  .3  to  3.75  mm.,  being  thicker 
on  the  back  of  the  head,  the  neck,  and  the  trunk  than  on  their 


262 


NORMAL   HISTOLOGY. 


Fig.  300. 


anterior  aspects,  and  thicker  on  the  outer  side  of  the  limbs  than  on 
their  mesial  surfaces. 

The  epidermis,  or  the  cuticle,  is  a  highly  developed  stratified 
squamous  epithelium  ;  while  forming  a  protecting  layer  to  the 
underlying  sensitive  corium  over 
the  entire  surface  of  the  body,  the 
epidermis  varies  in  different  regions, 
in  some  places,  as  on  the  eyelids 
and  brow,  not  exceeding  .  1  mm.  in 
thickness,  while  in  others,  as  on  the 
soles  of  the  feet  and  the  palms  of 
the  hands,  it  reaches  almost  1  mm. 
The  epidermis  is  accurately  adapted 
to  the  opposed  surface  of  the  corium, 
which  is  beset  with  papillae,  so  that 
when  the  two  layers  are  separated 


«?%^%%?^ 


m 


\'-\:V!p'.'.\:'^:#^'':.    ■'.'.  :-;!\-;-'*::'  '•' .:-y}?/-^'-:.''-~*';:'/.:.'{''''-;x':-:±Q-:-: 


Epidermis  of  human  skin  separated  from  corium, 
viewed  from  beneath  :  a,  thickened  areas  filling  de- 
pressions between  papillae ;  b,  pits  receiving  papillae 
of  corium  ;  c,  ducts  of  sweat-glands. 


Section  of  human  skin  from  hand,  in- 
cluding superficial  layer  of  corium  and 
epidermis :  a,  i,  c,  d,  respectively  the 
stratum  corneum,  lucidum,  granulosum,  and 
Malpighii ;  e,  layer  of  columnar  cells  next 
the  corium ;  /,  fibro-elastic  tissue  consti- 
tuting papillary  layer  of  corium. 


the  under  surface  of  the  epidermis  presents  impressions  or  pits 
corresponding  to  the  elevations  of  the  corium  which  they  receive. 

The  cells  composing  the  epidermis  are  arranged  in  many 
irregular  layers,  the  number  of  which  depends  upon  the  cuticular 
development  in  any  particular  region  ;  where  well  represented  the 
layers  are  grouped  into  two  sharply-defined  zones,  the  inner,  darker, 
softer  stratum  Malpighii  and  the  outer,  clearer,  denser  stratum 
corneum ;  where  highly  developed  the  epidermis  presents  two 
additional  zones,  distinguished  by  the  peculiar  character  of  the  pro- 
toplasm of  their  cells ;  these  layers  are  the  stratum  granulosum 
and  the  stratum  lucidum. 

The  stratum  Malpighii,  or  rete  mucosum,  contains  the  most 


THE   SKIN   AND    ITS   APPENDAGES.  26\ 

recently  formed  and  most  actively  growing  elements,  the  deepest  of 
which,  next  the  corium,  are  perpendicularly  placed  and  possess  a 
distinct  columnar  character.  The  irregular  and  often  slightly 
expanded  bases  of  the  deepest  cells  rest  upon  the  thin  basement- 
membrane,  while  their  outer  ends  are  surrounded  by  the  more  poly- 
hedral elements. 

Next  the  layer  of  columnar  cells  the  elements  become  broader  and 
polyhedral  in  form  and  possess  the  delicate  protoplasmic  spines 
characteristic  of  prickle-cells. 

The  elements  of  the  succeeding  horny  layer  stand  in  marked 
contrast  to  those  of  the  soft  underlying  Malpighian  stratum,  owing 
to  the  production  of  keratin  within  the  protoplasm  and  the  desicca- 
tion of  the  cells.  These  influences  are  seen  in  the  superficial  layers, 
in  the  disappearance  of  the  nucleus,  and  in  the  reduction  of  the  once 
large  polyhedral  cells  into  the  thin  compressed  horny  plates  of  the 
outer  layer. 

At  the  inner  border  of  the  horny  layer  lies  a  thin  band  of  cells, 
conspicuous  on  account  of  the  marked  granular  appearance  of  their 
protoplasm  ;  these  constitute  the  stratum  granulosum,  and  con- 
tain granules  of  eleidin,  a  peculiar  substance,  staining  intensely  in 
certain  dyes,  and  bearing  a  close  relation  to  the  keratin  of  the  more 
superficial  layers. 

At  the  outer  border  of  the  granular  stratum  the  horny  elements 
begin ;  those  lying  next  the  stratum  granulosum,  however,  are  in- 
completely transformed  into  horny  substance,  and  appear  as  an  ill- 
defined  narrow  zone,  the  stratum  lucidum,  which  contrasts  strongly 
with  the  darker  granular  layer.  Superficial  to  the  clear  zone  lie  the 
characteristic  cells  of  the  stratum  corneum  ;  these  epithelial  ele- 
ments are  enlarged  and  without  nuclei,  the  outermost  cells  being 
compressed  flattened  horny  scales,  which  after  desiccation  un- 
dergo desquamation  and  mechanical  abrasion. 

Over  those  parts  of  the  cutaneous  surface  where  the  epidermis  is 
well  developed  and  destitute  of  hairs,  the  stratum  corneum  differs 
somewhat  from  its  usual  condition  in  being  composed  chiefly  of  large 
distended  bladder-like  cells,  which  probably  represent  the  superficial 
epitrichial  layer  of  the  embryonal  skin.  Where  the  epidermis  is 
thin  the  stratum  granulosum  is  very  imperfect,  while  the  stratum 
lucidum  is  wanting ;  under  these  conditions  the  superficial  cells 
rapidly  dry  and  become  thin  horny  plates. 

Pigment-granules  are  widely  distributed  throughout  the  epider- 
mis, but  it  is  especially  within  the  deeper  layers  of  the  stratum  Mal- 
pighii  that  the  larger  accumulations  are  found  to  which  the  dusky  hue 
of  the  skin  of  many  races  is  due.  The  pigment-granules  do  not  origi- 
nate within  the  epithelial  elements,  but  are  conveyed  to  the  epidermis 


Fig 


2g4  NORMAL   HISTOLOGY. 

through  the  agency  of  the  migratory  cells,  the  cutis  vera.  The  dark 
tint  of  the  skin  of  the  negro  and  of  other  colored  races  depends 
almost  entirely  upon  the  pigment  within  the  epidermis,  since  in 
the  adult  integument  the  subepithelial  tissue  contains  comparatively 

few  pigmented  cells.  While  micro- 
scopical examination  shows  the  pres- 
ence of  pigment  some  weeks  before 
birth,  the  dark  color  is  usually  not 
evident  until  a  day  or  two  after- 
wards, owing  to  the  opaque  layer 
of  moist  superficial  scales  which 
masks  the  underlying  colored  cells. 
The  corium,  derma,  or  true 
skin  consists  of  a  felt-work  of  bun- 
dles of  white  fibrous  connective  tis- 
sue, with  which  elastic  fibres  and 
non-striped  muscle  are  mingled  in 
varying  amounts.  The  corium  is 
densest  in  its  outer  part,  where  be- 
neath the  epidermis  it  is  beset  with 
papillae,  which  greatly  extend  the 
sensory  surface  and  form  the  prin- 
cipal organ  of  tactile  sensibility. 
The  deeper  parts  of  the  corium  are 
much  looser  in  structure,  since  the 
bundles  are  coarser  and  more 
loosely  disposed,  fading  away  into  the  subcutaneous  tissue.  These 
differences  have  led  to  the  recognition  within  the  corium  of  an  outer, 
denser  stratum  papillare  and  an  inner,  looser  stratum  reticulare  ; 
no  sharp  demarcation  exists  between  the  two,  the  papillary  layer 
blending  with  the  reticular,  while  the  latter  in  turn  passes  gradually 
into  the  tissues  of  the  subcutaneous  stratum. 

The  papillae  vary  in  size,  number,  and  disposition  in  different 
regions,  being  best  developed  and  most  numerous  on  the  palmar 
surface  of  the  hands  and  the  fingers  and  on  the  corresponding  parts 
of  the  feet,  where  they  attain  a  height  of  .25  mm.;  on  the  other 
hand,  the  papillae  may  be  very  slightly  developed  or  even  absent. 
These  elevations  consist  of  closely-arranged  bundles  of  fibro-elastic 
tissue,  and  support  the  vascular  loops  together  with  the  rich  ter- 
minal nerve-supply ;  in  certain  localities  the  latter  includes  the 
highly- specialized  tactile  corpuscles  of  Meissner,  the  corpuscles  of 
Vater,  and  the  various  end-bulbs  which  already  have  been  described 
in  Chapter  VI.  The  simplest  type  of  the  papillae  is  the  rounded 
or  blunted  conical  elevation,  but  very  often  such  projection  becomes 


pfgtl 


Section  of  negro's  skin,  including  epidermis 
(a)  and  papillary  layer  of  corium  (6)  ;  the 
deepest  layers  of  epidermis  (c)  contain  the 
pigment. 


THE  SKIN   AND   ITS   APPENDAGES.  2gc 

cleft  and  converted  into  one  of  the  compound  variety.  The 
papilla?  of  the  hand  and  the  foot  are  distributed  in  characteristi- 
cally-arranged rows,  which  form  elaborate,  and  for  each  individual 
constant  and  distinctive,  ridges  on  the  integumentary  surface.  These 
ridges  have  been  found  to  retain  their  definite  arrangement,  or 
"patterns,"  from  early  life  to  old  age  unchanged.  This  constancy 
in  the  details  of  the  surface  markings  has  been  taken  advantage  of 
in  securing  records  by  means  of  impressions  for  the  purposes  of 
identification. 

Non-striped  muscular  tissue  occurs  within  the  corium  in  con- 
nection with  the  hair-follicles,  as  the  arrectores  pilorum,  and  in  the 
subcutaneous  tissue,  attached  to  the  under  surface  of  the  corium, 
in  particular  localities,  as  in  the  scrotum,  the  perineum,  the  penis, 
and  in  and  around  the  nipple. 

The  subcutaneous  stratum  consists  of  a  reticular  framework  of 
loosely-disposed  fibro-elastic  bundles  continued  from  those  of  the 
deeper  layers  of  the  corium  without  sensible  interruption  ;  the  inter- 
fascicular spaces  are  largely  occupied  by  adipose  tissue,  which 
in  many  places  forms  a  compact  layer,  the  panniculus  adiposus. 
The  cellular  elements  of  the  subcutaneous  tissue  are  the  usual  cells 
of  connective  tissue,  including  fusiform  and  plate-like  elements, 
leucocytes,  and  fat-cells  :  while  the  latter  are  quite  constant  con- 
stituents of  the  deeper  layers  of  the  skin,  within  the  integument  of 
the  eyelids,  the  penis,  and  the  labia  minora  fat  does  not  occur. 

THE   APPENDAGES    OF   THE    SKIN. 

These  include  the  nails  and  the  hairs,  together  with  the  cutane- 
ous glands,  all  of  which  are  directly  derived  from  the  ectodermic 
epithelium  of  the  epidermis. 

THE    NAILS. 

Each  nail  consists  of  a  large  exposed  body,  which  ends  ante- 
riorly in  the  projecting  free  edge,  and  extends  posteriorly  as  the 
root  some  considerable  distance  beneath  the  overhanging  upper 
margin  of  the  groove,  or  nail-fold,  receiving  the  root ;  at  the  sides 
the  borders  of  the  nail  are  covered  by  the  nail-walls.  The  nail, 
which  represents  an  enormously  developed  stratum  lucidum,  rests 
upon  a  highly  vascular  and  sensitive  nail-bed,  the  posterior  portion 
of  which,  covered  by  the  root  of  the  nail,  is  the  matrix.  The  nail- 
root  is  usually  lighter  in  color  and  somewhat  opaque,  owing  to  the 
thickness  of  the  stratum  Malpighii ;  on  the  thumb  it  extends  beyond 
the  nail-fold  as  a  pale  projecting  convex  area,  the  lunula. 

While  attached  throughout  the  extent  of  the  nail-bed,  the  growth 
of  the  nail  takes  place  from  the  matrix  alone,  each  newly-formed 


266 


NORMAL   HISTOLOGY. 


increment  pushing  before  it  the  older  already  existing  parts  at  the 
rate  of  about  one  millimetre  per  week. 

The  nail-bed  comprises  the  corium  and  that  portion  of  the 
epidermis  corresponding  to  the  stratum  Malpighii.  The  corium 
consists  of  the  usual  bundles  of  fibro-elastic  tissue,  which  are  arranged 
somewhat  parallel  to  the  long  axis  of  the  finger,  the  longitudinal  bun- 
dles being  supplemented  by  vertical  ones  extending  from  the  perios- 
teum of  the  phalanx  to  the  superficial  layers.    The  minute  elevations 

Fig.  "?02. 


Transverse  section  of  child's  finger,  including  the  nail :  a,  connective  tissue  of  corium ;  b,  longi- 
tudinally corrugated  nail-bed  ;  c,  corneous  tissue  constituting  body  of  nail ;  d,  its  thin  edge  covered 
by  tissue  of  nail-wall  (_/")  ;  e,  point  where  stratum  Malpighii  becomes  continuous  with  nail-bed. 

which  occupy  the  surface  of  the  corium  in  transverse  section  are  not 
true  papillae,  except  posteriorly  over  the  matrix,  but  longitudinal 
ridges.  They  are  lowest  behind  and  gradually  increase  in  height 
towards  the  front  of  the  nail,  terminating  abruptly  at  the  point  where 
the  latter  parts  from  its  bed.  The  epithelial  portion  of  the  nail- 
bed  is  principally  composed  of  cells  belonging  to  the  stratum  Mal- 
pighii, whose  numerous  layers  fill  up  the  inequalities  between  the 
papillae  and  the  ridges  of  the  corium  below,  and  are  sharply  defined 
from  the  substance  of  the  nail  above.  The  transformation  of  the 
deeper  cells  into  the  horny  plates  of  the  nail  takes  place  only  over 
the  matrix,  where  the  constantly-recurring  division  of  the  epithe- 
lial elements  furnishes  material  for  the  growth  of  the  nail.  The 
nail-fold  and  the  nail-wall  have  the  same  general  structure  as  the 
skin. 

The  substance  of  the  nail  itself  consists  of  intimately  united 
lamellae  of  horny  epithelial  cells,  which  possess  a  nucleus  and 
closely  resemble  the  elements  of  the  stratum  lucidum  ;  the  older 
and  most  superficial  layers  are  made  up  of  compressed  horny  dry 
scales,  while  those  composing  the  last  formed  and  hence  deepest  layer 
are  softer  and  more  regularly  polyhedral,  resembling  the  cells  of  the 
stratum  Malpighii. 


THE  SKIN  AND   ITS   APPENDAGES. 


267 


THE   HAIR. 

The  hairs,  together  with  their  homologues,  feathers  and  scales  of 
the  lower  animals,  are  derived  entirely  from  the  epidermis,  and 
are  therefore  of  ectodermic  origin.  These  slender  flexible  horny 
threads  are  distributed,  with  few  exceptions,  over  the  entire  surface, 
but  differ  greatly  as  to  both  size  and  frequency  in  various  regions ; 
individual  and  race  peculiarities  also  greatly  influence  the  character 
of  the  hair.  In  general,  in  straight-haired  races  the  hairs  are 
thicker  and  coarser  and  more  cylindrical  than  in  crisp-haired  races  ; 


Fig.  303 


Section  of  human  scalp,  showing  hair-follicles  and  sebaceous  glands  :  a,  epi- 
dermis ;  b,  corium ;  c,  hair  embraced  within  its  hair-follicle ;  d,  fibrous  sheath 
of  follicle  ;  e,  glassy  membrane ;  /,  outer  root-sheath  ;  g,  inner  root-sheath  ;  h, 
expanded  terminal  bulb  of  hair  ;  i,  hair-papilla;  k,  mouth  of  follicle  from  which 
hair-shaft  (/)  projects  ;  m,  adipose  tissue  ;  n,  blood-vessel ;  o,  sebaceous  glands  ; 
/,  arrector  pili  muscle  ;  s,  portions  of  sweat-gland. 

in  the  negro  the  hairs  are  flattened  cylinders,  small  and  oval  in  sec- 
tion ;  dark  hair  is  usually  coarser  than  that  of  light  color. 

Every  hair  presents  two  principal  divisions,  the  part  which 
projects  beyond  the  surface,  as  the  shaft,  and  the  portion  embedded 


268 


NORMAL   HISTOLOGY. 


Fig.  304. 


within  the  integument,  the  root ;  at  its  lower  extremity  the  root  ter- 
minates in  a  bulbous  expansion,  the  hair-bulb,  which  at  its  lowest 
point  is  indented  to  receive  the  connective-tissue  papilla.  The  hair- 
bulb  lies  embraced  within  a  pocket  of  modified  integument,  the 
hair-follicle,  to  which  the  corium  and  the  epidermis  contribute 
respectively  the  fibrous  and  the  epithelial  root-sheaths. 

The  hair  consists  entirely  of  epithelial  cells  disposed  as  three  dis- 
tinctly defined  strata,  the  cuticle,  the  cortical  substance,  and  the 
medulla,  or  pith.  The  hair  cuticle  is  composed  of  a  single  layer 
of  thin,  horny,  imbricated  scales,  which  envelop  the  entire  sur- 
face of  the  hair,  both  on  the  root  and  on  the  shaft ;  in  these  situations 
it  forms  a  layer  respectively  6-8  fi  and  2-4  11  in  thickness.  Owing 
to  the  imbricated  arrangement  of  the  cells,  as  tiles  upon  a  roof,  only 
their  free  projecting  borders  are  visible,  which  produce  in  surface  views 

the  characteristic  oblique  transverse 
markings  so  distinctive  of  hair;  in  pro- 
file the  edges  of  the  cells  appear  as  deli- 
cate serrations. 

The  cortical  substance  constitutes  by 
far  the  greater  part  of  the  hair,  when  the 
medulla  is  wanting  sometimes  forming  its 
entire  bulk.  This  portion  of  the  hair- 
shaft  is  composed  of  greatly  elongated 
horny  epithelial  cells,  which  possess 
attenuated  nuclei  and  are  so  intimately 
united  that  the  boundaries  of  the  individ- 
ual elements  under  ordinary  circumstances 
are  not  distinguishable.  On  the  root  the 
cells  are  broader,  less  horny,  and  assume 
more  and  more  the  character  of  the  ele- 
ments of  the  stratum  mucosum  as  the  prox- 
imal end  of  the  hair-bulb  is  approached  ; 
immediately  around  the  papilla  the 
cells  of  the  cortical  substance  become  con- 
tinuous with  the  extension  of  the  stratum  mucosum,  the  outer  root- 
sheath. 

The  medulla,  or  pith,  occupies  the  central  tract  of  the  hair-shaft, 
and  extends  in  favorable  examples  from  near  the  hair-bulb  almost  to 
the  extremity  of  the  hair.  Many  hairs  possess  no  pith,  this  part 
being  usually  wanting  in  the  fine  hairs  of  the  general  body-surface 
and  the  colored  hairs  of  the  head,  as  well  as  in  the  hairs  of  chil- 
dren under  four  or  five  years  of  age.  In  the  thick  short  and  in  the 
robust  long  hairs,  likewise  in  most  white  scalp-hairs,  the  medulla  is 
present,  and  constitutes  sometimes  one-third  of  the  diameter  of  the  hair. 


A,  human  hair;  the  upper  half 
of  the  figure  represents  the  super- 
ficial horny  cells  (k)  constituting  the 
cuticle,  the  lower  half,  the  fibrous 
structure  of  the  cortical  substance 
and  the  medulla  ;  B,  isolated  ele- 
ments of  the  hair ;  a,  cuticular 
scales  ;  5,  thin  fibre-cells  of  cortical 
substance. 


Fig.  305. 

111! 


THE   SKIN   AND   ITS   APPENDAGES.  26q 

The  medulla  is  composed  of  rows  of  irregular  cuboidal  or 
spherical  cells,  15-20  y.  in  diameter,  filled  with  dark  granules,  which 
really  are  minute  air-vesicles  ;  by  reflected  light  the  pith  appears 
silvery-white,  while  by  transmitted  light  it  is  dark  and  opaque.  The 
air  gains  access  to  the  medulla  in  consequence  of  the  partial  drying 
out  of  the  soft  protoplasm  of  the  cells.  In  many  animals  the  medulla- 
cells  form  a  conspicuous  and  relatively  large  portion  of  the  hair,  and 
present  characteristic  details  sufficiently  dis- 
tinctive to  determine  the  kind  of  animal  from 
which  the  specimen  was  obtained. 

The  color  of  the  hair  depends  upon  the 
presence  of  pigment-granules,  diffuse  pig- 
ment, and  air.  The  granular  pigment 
occurs  as  colored  particles  varying  from  light 
brown  to  black  ;  in  dark  hair  the  pigment  lies 
within  the  elements  of  the  cortical  substance,  as 
well  as  often  between  the  cells,  the  cortex  in 
addition  sometimes  containing  diffuse  soluble 
coloring-matter  in  combination  with  the  proto- 
plasm of  the  cells.  Diffuse  pigment  is  en- 
tirely wanting  in  white  hair,  is  sparingly  present 
in  light  blond  hair,  and  exists  in  abundance  in 
dark  blond,  red,  and  dark  hair. 

The  hair-follicles  are  tubular  or  flask-shaped 
depressions  within  the  integument  (2-7  mm.  in 
length)  which  tightly  embrace  the  hair-shafts  ; 
those  of  the  finer  hairs  lie  entirely  within  the 
corium,  while  those  of  the  large  hairs  frequently 
extend  deeply  into  the  subcutaneous  stratum. 

The  hair-follicle  serves  the  double  purpose  of 
supplying  the  tissue  from  which  the  hair  is  formed 
and  of  affording  the  necessary  attachment  and 
support  to  the  hair  after  its  development.  The 
relation  of  the  hair-follicle  to  the  general  integu- 
ment is  best  appreciated  by  remembering  that 
the  follicle  develops  by  an  ingrowth  of  the 
epidermis  into  the  subjacent  connective  tis- 
sue ;  the  hair  subsequently  appears  as  the  result 
of  the  metamorphosis  and  the  differentiation  of 
the  cells  occupying  the  most  dependent  part  of 
the  epidermal  plug.  While  in  the  follicles  of  the 
finer  hairs  the  epithelium  forms  almost  the  entire  structure,  in  those 
of  the  large  hairs  the  surrounding  connective  tissue  takes  part  to  the 
extent  of  supplying  a  strong  protective  sheath,  the  fibrous  coat. 


Hair-follicle  from  human 
scalp :  a,  hair ;  b,  inner 
root-sheath ;  c,  outer  root- 
sheath  ;  d,  glassy  mem- 
brane ;  e,  fibrous  sheath  ; 
f,  hair-bulb ;  h,  hair-pa- 
pilla. 


270 


NORMAL    HISTOLOGY. 


Below  the  openings  of  the  sebaceous  glands  the  hair-follicle 
consists  of  the  fibrous  coat  and  the  stratum  mucosum  of  the  epi- 
dermis only  :  at  its  upper  extremity  the  stratum  corneum  additionally 
takes  part  in  the  formation  of  the  follicle.  The  fibrous  coat  of  the 
follicle  consists  of  three  layers  :  the  outer,  composed  of  longitudinally- 
placed  bundles  of  connective  tissue,  rich  in  cells,  and  representing  a 
condensation  of  the  tissue  of  the  corium  ;  the  middle,  represented 
by  a  layer  of  circular  connective-tissue  bundles  continuous  with  the 
papillary  layer  of  the  cutis ;  and  the  inner,  a  clear,  homogeneous, 
narrow  but  conspicuous  zone,  the  glassy  or  hyaline  membrane. 
The  latter  separates  the  epithelium  from  the  surrounding  fibrous 
tissue,  and  corresponds  to  a  highly-developed  basement-mem- 
brane. These  layers  of  the  fibrous  sheath  are  not  continued  to  an 
equal  extent  over  the  hair-follicle  ;  the  outer  longitudinal  layer  com- 
pletely invests  the  follicle,  becomes  continuous  with  the  corium,  and 
materially  aids  in  maintaining  the  form  of  the  follicle.     The  circular 


Fig.  w. 


Fig.  306. 


Transverse  sections  of  hair-follicles 
from  human  scalp  :  a,  hair ;  b,  cuticle 
of  hair ;  c,  d,  inner  and  outer  root- 
sheath  ;  e,  glassy  membrane  ;_/",  fibrous 
sheath ;  g-,  surrounding  connective 
tissue  of  corium  ;  h,  fat-cells. 


Transverse  section  of  hair-follicle  from  human  scalp  ; 
plane  of  section  passes  through  mouth  of  follicle  :  a, 
one  of  the  hairs;  b,  horny  tissue  of  superficial  layers 
of  epidermis  ;  c,  cells  of  stratum  Malpighii ;  d,  sur- 
rounding connective  tissue. 


layer  extends  from  the  base  of  the  hair-follicle  to  the  orifices  of  the 
sebaceous  glands,  while  the  glassy  membrane,  as  such,  ceases  at  the 
mouth  of  the  follicle. 

Next  inside  the  glassy  membrane  follow  the  epithelial  layers  occu- 
pying the  entire  space  between  the  hair  and  the  sides  of  its  follicle. 
The  epithelial  tissue  is  disposed  in  two  well-marked  strata,  the 
thicker,  many-layered  zone  next  the  glassy  membrane,  which  con- 
stitutes the  outer  root-sheath,  and  the  much  thinner  concentric 
layer  composing  the  inner  root-sheath.  The  former  is  the  direct 
prolongation  of  the  stratum  mucosum  of  the  general  integu- 


THE   SKIN  AND   ITS   APPENDAGES.  271 

ment,  while  the  latter  is  derived  from  a  part  of  the  same  cells  that 
form  the  hair  itself,  and  is  therefore  closely  related  to  the  hair. 

The  outer  root-sheath  being  the  direct  continuation  of  the 
stratum  mucosum  of  the  adjacent  skin,  its  structure  corresponds 
with  that  layer  of  the  epidermis  ;  when  well  developed,  as  in  the 
follicles  of  the  larger  hairs,  this  sheath  measures  40-60  ft  in  thick- 
ness, or  more  than  twice  the  breadth  of  the  inner  root-sheath.  In 
the  upper  part  of  the  follicle,  where  the  glassy  membrane  and  the 
circular  fibrous  layer  are  wanting,  the  outer  sheath  rests  directly  in 
contact  with  the  longitudinal  layer.     The  inner  cells  of  the  root- 


M^J&&\* 


Transverse  section  of  hair-follicle  from  human  scalp,  more  highly  magnified :  a,  substance  of  hair, 
condensed  at  periphery  {b) ;  c,  cuticular  layer,  composed  of  cuticles  of  hair  and  of  inner  root-sheath ; 
d,  e,  respectively  layer  of  Huxley  and  of  Henle ;  f,  outer  root-sheath  ;  g,  glassy  membrane ;  h,  i, 
circular  and  longitudinal  bundles  of  fibrous  sheath. 


sheath  are  columnar  and  placed  vertically  upon  the  glassy  mem- 
brane, while  the  cells  of  the  succeeding  layers,  some  five  to  ten 
deep,  present  the  polygonal  outlines  and  the  intercellular  connect- 
ing threads  seen  in  the  corresponding  parts  of  the  ordinary  epider- 
mis. The  space  between  the  outer  root-sheath  and  the  hair  is  occu- 
pied by  three  narrow  zones,  which  collectively  form  the  inner 
root-sheath,  a  clear  transparent  rigid  membrane  closely  embracing 
the  lower  two-thirds  of  the  hair-follicle  and  terminating  in  the  vicinity 
of  the  opening  of  the  sebaceous  gland.  The  outer  or  Henle 's 
layer  appears  as  a  light  band  composed  of  somewhat  elongated 


272  NORMAL   HISTOLOGY. 

polyhedral  cells,  whose  protoplasm  is  very  faintly  granular  and  whose 
nuclei  are  wanting.  Next  follows  Huxley's  layer,  consisting  of  a 
single  or  double  row  of  shorter  and  broader  polyhedral  cells,  which 
ordinarily  display  small  nuclei ;  at  the  lower  part  of  the  follicle  these 
cells  contain  numerous  granules,  probably  of  eleidin.  Of  the  15-35 
/j.  representing  the  entire  thickness  of  the  inner  root-sheath,  Henle's 
layer  contributes  about  one-third,  the  remaining  two-thirds  being 
made  up  by  the  layer  of  Huxley.  The  outer  surface  of  Huxley's 
layer  is  covered  with  the  clear  delicate  cuticle  of  the  root-sheath, 
a  single  layer  of  thin  transparent  plates  lying  against  the  cuticle  of 
the  hair  in  such  close  relation  that  the  two  cuticular  layers  appear  as 
one.  The  cells  of  this  envelope  are  imbricated  in  a  manner  similar 
to  those  of  the  hair-cuticle,  but  the  free  edges  of  the  plates  are  di- 
rected in  the  opposite  direction  from  those  of  the  hair,  the  serrations 
of  the  cuticle  of  the  root-sheath  fitting  into  the  impressions  on  the 
surface  of  the  hair. 

The  extremity  or  base  of  the  hair-follicle  presents  a  deep  invagina- 
tion for  the  reception  of  the  process  of  dermal  connective  tissue  con- 
stituting the  hair-papilla.  The  latter  is  a  large,  simple,  club-shaped 
elevation,  .1— .3  mm.  in  length,  which  usually  contains  numerous 
branched  pigment-cells  and  loops  of  blood-vessels.  The  presence 
of  nerves  within  the  papillae,  on  the  contrary,  is  very  doubtful. 

The  most  interesting  as  well  as  important  part  of  the  hair-follicle 
is  immediately  around  the  hair-papillae,  since  to  the  differentia- 
tion of  the  soft  granular  polyhedral  cells  occupying  this  position  the 
hair,  together  with  the  inner  root-sheath,  owes  its  formation.  These 
elements  are  the  direct  derivatives  of  the  stratum  mucosum, 
and  represent  the  centre  of  greatest  activity ;  the  elements  di- 
rectly over  the  papilla  supply  the  material  from  which  the  hair  proper 
is  developed,  while  the  cells  at  the  lower  part  of  its  sides  become 
transformed  into  the  layers  of  the  inner  root-sheath.  For  some  dis- 
tance immediately  above  the  summit  of  the  papilla,  polyhedral  nu- 
cleated granular,  and  often  pigmented,  cells  compose  a  matrix  from 
which  the  constituents  of  the  cortical  and  medullary  portions  of  the 
hair  are  directly  derived. 

The  muscles  of  the  hairs,  the  arrectores  pilorum,  exist  as 
minute  flattened  plexiform  bundles  of  non-striped  muscle,  which 
extend  from  the  most  superficial  parts  of  the  corium  to  the  hair- 
follicles  ;  the  muscular  band  is  attached  to  the  fibrous  coat  of  the 
follicle,  below  the  sebaceous  glands,  on  the  side  towards  which  the 
hair  is  directed.  When  the  muscle  contracts  the  obliquely-placed 
follicle  becomes  perpendicular  and  the  shaft  erect,  in  consequence  of 
which  the  integument  attached  about  the  hair  is  drawn  up,  producing 
the  well-known   condition  of  cutis   anserina,   or   "goose-flesh." 


THE   SKIN   AND   ITS   APPENDAGES. 


273 


Muscular  slips  frequently  encircle  the  lower  part  of  the  follicle,  while 
additional  bands  sometimes  are  given  off  to  find  attachment  in  the 
fibrous  sheath  of  the  sweat-glands. 


THE   SEBACEOUS    GLANDS. 

These  structures  occur  so  closely  connected  with  the  hair- 
follicles,  into  which  they  usually  open,  that  the  sebaceous  glands 
may  be  looked  for  wherever  hairs  exist ;  in  addition,  the  glands  may 
be  present  when  hair-follicles  are  absent,  as  on  the  external  genitalia 
(labia  minora,  glans  and  prepuce  of  the  penis),  the  eyelids  (Mei- 
bomian glands),  and  the  red  edge  of  the  lips.  The  smallest  se- 
baceous glands  are  connected  with  the  head-hairs,  while  the  largest 
are  found  on  the  mons  Veneris,  the  labia 
majora,  and  the  scrotum.  The  size  of  these 
structures  is  not  proportionate  to  that  of 
the  associated  hairs,  since  frequently  the  fine 
lanugo  hairs  possess  large  glands,  a  relation 
also  seen  in  the  particularly  well  developed 
sebaceous  sacs  connected  with  the  fine  hairs 
on  the  nose  and  the  face.  The  group  of  acini 
is  usually  placed  on  the  side  towards  which 
the  hairs  slope,  and  occupies  the  interval  be- 
tween the  hair-follicle  and  the  arrector  pili 
muscle,  the  contractions  of  the  latter  aiding 
in  the  expulsion  of  the  secretion  of  the  gland. 

The  sebaceous  glands  are  sometimes 
simple  but  usually  small  compound  saccular 
structures  possessing  short  ducts  which 
open  into  the  hair-follicles  near  their  upper 
extremities.  The  periphery  of  the  acini, 
five  to  twenty  in  number,  is  lined  by  a  pe- 
ripheral layer  of  cuboidal  epithelium,  while 
the  greater  part  of  the  sacs  is  filled  with 
cells  in  various  stages  of  fatty  metamor- 
phosis. 

The  secretion  of  these  glands,  the  sebum, 
when   fresh   at  the   body-temperature,   is   a 

semi-fluid  substance  consisting  of  oil-droplets  and  the  debris  of 
broken-down  cells ;  on  exposure  to  the  atmosphere  it  becomes  of 
the  consistence  of  tallow. 


Section  of  portion  of  seba- 
ceous gland  from  human  scalp, 
including  part  of  acinus  :  a, 
membrana  propria ;  b,  periph- 
eral layer  of  cuboidal  cells  ;  c, 
elements  in  which  fatty  meta- 
morphosis is  beginning  ;  d,  cells 
filled  with  fatty  particles  and 
exhibiting  marked  intra-cellular 
net-works  ;  e,  nuclei  of  cells. 


THE   SWEAT-GLANDS. 


The  sweat  or  sudoriparous  glands  are  modified  simple  tubular 
glands  which  extend  from  the  free  surface  of  the  integument,  where 


274  NORMAL   HISTOLOGY. 

they  open  by  the  trumpet-shaped  orifices  of  their  wavy  ducts  to  the 
deepest  part  of  the  reticular  layer  of  the  corium,  or  still  farther  into 
the  subcutaneous  stratum,  in  which  position  the  gland-tube  ends  as 
a  greatly  convoluted  spherical  mass.  The  sweat-glands  enjoy  a  very 
wide  distribution,  being  present  in  greater  or  less  abundance  over 
the  entire  body-surface,  with  the  exception  of  the  deeper  parts  of 
the  external  auditory  canal  and  the  tympanic  membrane. 

The  largest  sweat-glands  occur  in  the  axilla,  at  the  root  of  the 
penis,  on  the  labia  majora,  and  around  the  anus.  While  the  average 
diameter  of  the  gland-masses  is  .3-. 4  mm.,  the  axillary  glands 
'  measure  2-7  mm.  at  their  widest  part.  Each  sweat-gland  presents 
two  divisions,  the  greatly  convoluted  gland-coil  and  the  much 
straighter,  slightly  wavy  excretory  duct ;  the  former,  which  repre- 
sents the  secreting  portion  of  the  gland,  is  much  wider,  both  in 
its  general  diameter  and  lumen,  than  the  part  constituting  the  duct. 
The  gland-tube  is  limited  by  a  membrana  propria  continuous  with 
that  of  the  skin,  outside  of  which  a  delicate  connective-tissue  en- 
velope gives  additional  strength  ;  within  the  basement-membrane 
cuboidal  or  low  columnar  epithelial  cells  form  the  lining  of  all 
parts  of  the  gland.  In  the  secreting  division  of  the  tube  the  low 
columnar  cells  are  disposed  as  a  single  stratum,  while  those  lining 
the  duct  are  arranged  as  a  double  layer  of  small  and  low  polygonal 
elements ;  the  cells  of  the  duct  are  covered  next  the  lumen  of  the 
tube  with  a  delicate  cuticle. 

The  duct  from  the  secretory  portion  of  the  gland  to  the  epidermis 
maintains  an  almost  constant  diameter  (20-25  i±)  ;  on  entering  the 
epidermis,  however,  it  enlarges  to  almost  double,  and  on  reaching 
the  stratum  corium  expands  into  the  trumpet-shaped  orifice  which 
marks  its  termination.  Within  the  epidermis  the  duct  loses  its 
distinct  walls,  the  final  turns  of  its  spiral,  corkscrew-like  course 
being  bounded  by  the  horny  plates  of  the  epidermis.  In  ex- 
ceptional cases  the  sweat-glands  open  into  the  upper  part  of  the 
hair-follicles,  but,  as  a  rule,  they  reach  the  free  surface  by  entering 
the  epidermis  in  the  depressions  between  the  papillae  of  the 
corium. 

The  terminal  secretory  segment  of  the  gland-tube,  usually 
single,  although  sometimes  branched,  is  convoluted  to  form  the  char- 
acteristic coils,  which  can  be  seen  often  with  the  unaided  eye  as 
reddish-yellow  spherical  masses.  The  columnar  secreting  cells 
(10-20  fi  in  height)  present  a  single  layer  of  elements  whose  pro- 
toplasm is  very  finely  granular  and  sometimes  contains  fatty  gran- 
ules, as  well  as  yellow  or  brown  particles  ;  these  latter  are  espe- 
cially evident  in  the  ceruminous  glands  of  the  external  ear,  the  axillary 
and  the  mammary  areolar  glands.     The  nuclei  of  the  secreting  cells 


THE    SKIN   AND    ITS    APPENDAGES. 


275 


are  eccentrically  placed,  while  the  border  of  the  cells  next  the  lumen 
presents  a  thickened  edge  sometimes  described  as  a  cuticle. 

Immediately  outside  the  epithelial  cells,  between  these  and  the 
basement-membrane,   lies   a   thin 

layer  of  involuntary   muscle  ;  Fig.  310. 

this  tissue  occurs  only  in  the  se- 
cretory division  of  the  tube,  and 
is  best  developed  in  the  larger 
glands,  where  the  muscle  -  cells 
form  a  complete  layer.  The  in- 
dividual convolutions  of  the 
tube  constituting  the  coil  are  held 
together  by  delicate  connective 
tissue,  which  additionally  furnishes 
a  fibrous  envelope  for  the  entire 
mass.  The  average  diameter  of 
the  secreting  portion  of  the  gland 
is  about  65  fi,  of  which  about  30  ft 
are  contributed  by  the  epithelial 
lining,  and  about  half  as  much  by 
the  fibrous  and  muscular  tunics  ; 
the  remaining  20  fi  represent  the 
usual  lumen. 

The  secretion  of  the  sweat- 
glands  varies  with  the  locality  and 
the  character  of  the  glands  ;  in 
general    the    secretion    of   these 

structures  occurs  in  two  forms, — as  the  colorless,  slightly  turbid 
fluid,  devoid  of  morphological  constituents,  which  is  elaborated  by 
the  smaller  glands  and  is  the  sweat  proper,  and  as  the  thicker  oily 
substances  supplied  by  the  large  axillary,  the  circumanal,  and  the 
ceruminous  glands.  The  products  of  these  structures  consist  mostly 
of  water,  but  contain,  in  addition,  about  1.2  per  cent,  of  solids,  in- 
cluding fat,  fat-acids,  albuminous  matters,  urea,  and  salts  in  various 
proportions  and  combinations. 

The  ceruminous  glands  of  the  ear  and  the  glands  of  Moll  of 
the  eyelid  must  be  regarded  as  modified  sudoriparous  glands,  since 
they  closely  correspond  to  the  sweat-tubes  in  structure. 

The  total  number  of  sweat-glands  of  the  human  body  has  been  esti- 
mated to  be  about  two  millions  (Krause)  ;  they  are  most  numerous 
on  the  palms  of  the  hands,  in  which  situation  373  occur  within  a  single 
square  centimetre,  and  are  almost  as  frequent  on  the  soles  of  the  feet  ; 
the  glands  are  most  sparingly  distributed  over  the  back  and  the  but- 
tocks, where  less  than  sixty  are  contained  within  one  square  centimetre. 


,  Section  of  coiled  part  of  sweat-gland  from 
human  skin  :  a,  a,  secreting  portion  of  tubule, 
cut  in  various  directions  ;  b,  b,  parts  representing 
beginning  of  duct;  c,  intertubular  conneciive 
tissue  ;  d,  layer  of  involuntary  muscle  inside  the 
basement-membrane  ;  e,  cuticular  border. 


2/6 


NORMAL   HISTOLOGY. 


BLOOD-VESSELS,  LYMPHATICS,  AND    NERVES   OF   THE   SKIN. 

The  blood-vessels  supplying  the  skin  are  arranged  as  three  sets, 
which  occupy  different  levels,  and  are  destined  especially  for  the 
structures  lying  within  the  respective  layers.  The  larger  arterial 
vessels  run  between  the  superficial  fasciae  and  the  integument, 
generally  parallel  to  the  latter,  while  perpendicular  branches  are  given 
off  which  pass  towards  the  free  surface  and  early  in  their  course 
provide  twigs  for  the  supply  of  the  deep-lying  fat-clusters,  among 
which  the  arterioles  break  up  into  the  capillary  net-works.  At  a 
somewhat  higher  level  branches  are  given  off  to  the  sweat-glands, 
superficially  to  which  a  net- work  is  formed  by  the  terminal  branches 
of  the  ascending  arteries,  and  constitutes  a  rich  subepithelial 
reticulum  distributed  to  the  outermost  stratum  of  the  corium. 
Where  well  developed,  the  papillae  receive  vascular  tufts  and  loops 
from  the  subepithelial  net-work,  the  disposition  of  the  loops  corre- 
sponding with  the  simple  or  compound  character  of  the  papillae. 
Numerous  twigs  also  provide  for  the  nutrition  of  the  hair-follicles, 
around  which  the  longitudinal  arterioles  are  connected  by  the  trans- 
versely-disposed capillary  net-works  encircling  the  follicles  ;  loops  are 
given  off  to  supply  the  hair-papillae,  as  well  as  small  branches  to  the 
sebaceous  glands  and  the  hair-muscles.  The  veins  follow  the  gen- 
eral arrangement  of  the  arterial  branches.  The  follicles  of  the 
conspicuous  tactile  hairs  of  the  lower  animals  are  surrounded  by 
the  large  venous  spaces  which  occupy  the  cavernous  tissue  situated 
between  the  longitudinal  and  the  circular  coat  of  the  fibrous  sheath. 

The  numerous  lymphatics  of  the  skin  are  arranged  in  two  gen- 
eral sets,  those  extending  within  the  corium  and  forming  the 
superficial  reticulum,  and  those  situated  within  the  subcutaneous 
tissue  and  following  the  larger  blood-vessels.  The  superficial 
lymphatics  begin  as  the  interfascicular  clefts  of  the  corium,  some  of 
which  are  contained  within  the  papillae ;  these  irregular  spaces,  with 
their  imperfect  lining  of  connective-tissue  plates,  communicate  with  the 
more  definite  lymph-vessels,  which  anastomose  to  form  the  plexus 
extending  throughout  the  corium  slightly  beneath  the  plane  of  the 
closer-meshed  reticulum  of  blood-capillaries.  Special  net-works  of 
lymph-capillaries  surround  the  hair-follicles  and  the  glands.  The 
deeper  set  of  lymphatics  lie  within  the  subcutaneous  tissue  and  con- 
stitute a  loose  reticulum  of  larger  vessels,  which  freely  communicate 
with  the  closer  superficial  lymphatic  net-works  as  well  as  with  those  sur- 
rounding the  adjacent  hair-follicles  and  the  glands.  Each  of  the  larger 
blood-vessels  is  usually  accompanied  by  two  lymphatic  trunks  of 
considerable  size,  which,  by  means  of  numerous  transverse  branches, 
freely  communicate  and  enclose  the  blood-vessels  within  their  meshes. 


THE    SKIN   AND    ITS    APPENDAGES.  277 

The  nerves  supplying  the  skin  vary  greatly  in  different  regions,  the 
palmar  surface  of  the  fingers  and  the  corresponding  parts  of  the  toes 
receiving  the  richest  supply.  The  larger  stems  lie  within  the  sub- 
cutaneous tissue,  from  which,  in  addition  to  twigs  distributed  directly 
to  the  sweat-glands  and  the  involuntary  muscle,  numerous 
branches  accompany  the  blood-vessels  into  the  corium  to  end  in 
various  ways.  Upon  reaching  the  superficial  portions  of  the  corium, 
after  having  given  off  many  lateral  branches,  the  ascending  twigs 
break  up  into  bundles,  which  form  a  rich  subpapillary  plexus,  con- 
taining both  medullated  and  pale  fibres,  and  extending  beneath  the 
epidermis  and  the  bases  of  the  papillae.  The  non-medullated 
fibres  are  probably  destined  for  the  involuntary  muscle  of  the  cutis, 
the  glands,  and  the  blood-vessels ;  the  medullated  fibres,  on  the 
other  hand,  are  connected  with  several  forms  of  special  nerve- 
endings.  From  the  superficial  plexus  within  the  corium  small  twigs 
ascend  to  the  epidermis,  some  fibres  ending  immediately  beneath 
the  epithelium,  while  others  pass  for  different  distances  between 
the  epithelial  elements  to  terminate  either  as  free  endings  or  in 
connection  with  the  tactile  cells.  The  branches  from  the  subpap- 
illary plexus  which  ascend  into  the  papillae  are  connected  with  the 
large  tactile  corpuscles  of  Meissner  which  occupy  the  non-vas- 
cular papillae.  Within  the  subcutaneous  layer,  in  many  regions, 
numerous  corpuscles  of  Vater  are  present.  The  hair-follicles 
receive  a  considerable  part  of  the  nerves  of  the  corium,  the  medul- 
lated fibres  forming  loose  net-works  around  the  follicles,  which  they 
accompany  as  far  as  the  sebaceous  glands,  where  they  divide  into  the 
naked  fibrillar  which  are  traceable  with  certainty  as  far  as  the  glassy 
membrane  and  probably  end  within  the  external  root-sheath. 


THE    DEVELOPMENT   OF   THE   SKIN   AND    ITS   APPENDAGES. 

The  development  of  the  skin  in-  Fig.  311. 

eludes  the  participation  of  the  ecto- 
derm and  the  mesoderm,  which  con- 
tribute respectively  the  epidermis  and 
the  corium.  The  history  of  the  epi- 
dermis is  closely  identified  with  that  of 
the  ectoderm.  In  the  earliest  stage  the 
latter  consists  of  a  single  layer  of 
low  cuboidal  cells ;  later  an  addi- 
tional superficial  stratum,  the  epi- 
trichium,  becomes  differentiated,  the 
two  layers  of  the  ectoderm  probably 
already  indicating  the  corneous  and 
Malpighian  strata  of  the  future  epidermis,  although  the  precise 


Section  of  developing  skin  from 
human  fcetus  of  three  and  a  half 
months  :  a,  layer  of  cuboidal  cells  rep- 
resenting stratum  Malpighii ;  b,  polyhe- 
dral elements  forming  superficial  layers ; 
c,  outermost  flattened  plates,  probably 
the  remains  of  the  epitrichial  layer ;  d, 
mesodermic  tissue  forming  corium. 


2yg  NORMAL   HISTOLOGY. 

relation  between  the  horny  layer  and  the  embryonal  cells  is  still  un- 
settled. It  is  probable  that  where  a  well-developed  stratum  corneum 
exists  the  parts  of  this  external  to  the  stratum  lucidum  represent  the 
metamorphosed  epitrichium  ;  where,  however,  a  true  cornified  layer 
is  wanting  and  the  superficial  cells  belong  to  a  highly-developed 
stratum  lucidum,  as  in  the  nails,  the  epitrichium  is  not  represented, 
since  in  this  case  the  entire  epidermis  is  derived  from  the  deeper 
layer  of  ectodermic  tissue  (Bowen,  Minot).  With  the  general 
growth  the  layers  of  the  epidermis  increase  in  number  and  the 
innermost  cells  assume  the  characteristic  columnar  character 
which  continues  distinctive  of  the  active  Malpighian  layer. 

The  corium  is  formed  of  mesodermic  tissue  which  becomes 
condensed  beneath  the  epithelial  layer  and  subsequently  is  beset 
with  numerous  papillary  elevations  ;  the  development  of  vascular 
structures  within  the  young  corium  takes  place  along  with  the  dif- 
ferentiation of  a  distinct  subepidermal  zone  within  the  mesoderm. 
Before  the  fourth  month  of  foetal  life  the  corium  and  the  subcu- 
taneous zones  have  become  defined,  and  a  little  later  fat-cells 
appear  within  the  last-named  layer. 

The  development  of  the  nails  depends  upon  the  specialization 
of  the  stratum  lucidum  within  certain  areas  connected  with  the  ter- 
minal phalanges.  The  earliest  indication  of  the  nail-formation  appears 
about  the  third  month  in  the  human  embryo,  and  consists  of  a  thick- 
ening of  the  primitive  epidermis  over  the  end  of  the  digit ;  the 
nail-area  becomes  defined  by  a  furrow  and  takes  up  a  permanent 
position  on  the  dorsal  aspect  of  the  finger,  when  an  ingrowth  of 
the  stratum  Malpighii  takes  place  to  establish  the  root  of  the 
nail.  About  the  fourth  month  the  upper  cells  of  the  Malpighian 
layer  exhibit  granules,  which  play  an  important  part  in  the  cornifi- 
cation  of  the  epithelial  elements  in  the  formation  of  the  nail ;  these 
granules  are  very  similar  to,  if  indeed  not  identical  with,  eleidin  in 
their  nature.  The  cells  of  the  stratum  lucidum  subsequently  un- 
dergo great  increase  and  constitute  the  body  of  the  nail.  Until  about 
the  fifth  month  the  young  nail  is  covered  superficially  by  the  epi- 
trichium, here  called  the  eponychium ;  the  latter  then  disappears,  and 
finally  is  represented  only  by  the  small  epithelial  band,  the  perionyx, 
which  persists  across  the  root  of  the  nail.  The  final  steps  in  the  nail- 
formation  are  associated  with  a  process  of  desquamation  of  the 
stratum  lucidum,  whereby  the  distal  end  of  the  nail  is  separated 
from  its  bed  and  the  existence  of  a  free  edge  is  established. 

By  the  addition  of  young  cells  at  its  posterior  margin  the  nail 
grows  in  length,  while  by  the  increments  to  its  under  surface 
derived  from  the  stratum  mucosum  at  the  lunula  it  increases  in 
thickness ;  the  thickest   part  of  the  nail  is,  therefore,  not  at  its 


THE   SKIN  AND   ITS   APPENDAGES.  27Q 

root,  but  at  the  anterior  border  of  the  lunula ;  from  this  point  for- 
ward the  nail  remains  of  constant  thickness,  since  it  derives  no  aug- 
mentation in  its  passage  over  the  nail-bed.     The  regeneration  of 


Fig.  312. 


Section  of  skin  of  foetal  kitten,  showing  earliest 
stage  of  development  of  hair :  a,  epidermis  ex- 
hibiting thickening  and  elevation  of  surface ;  b, 
mesodermic  tissue,  showing  indications  of  con- 
densation. 


Fig.  313. 


Section  of  skin  of  fcetal  kitten,  showing  ecto- 
dermic  tissue  (a)  starting  to  grow  into  mesoderm 
(6)  as  solid  epithelial  process. 


the  nail  after  disease  or  injury  depends  upon  the  integrity  of  the 
deeper  layers  of  the  epithelium. 

The  development  of  the  hair  in  the  foetus  proceeds  entirely 
from  the  ectoderm.  The  first  indication  of  the  process,  about  the 
end  of  the  third  month,  ap- 
pears as  a  localized  prolif-  FlG-  3*4- 
eration  of  the  ectodermic 
cells,  resulting  in  a  slight 
transient  elevation  of  the  sur- 
face, and,  at  the  same  time, 
in  a  feeble  encroachment  on 
the  subjacent  mesoderm. 
This  ectodermal  projection 
soon  becomes  an  epithelial 
cylinder,  whose  expanded 
club-shaped  extremity  pene- 
trates deeply  into  the  primi- 
tive corium  to  form  the  epi- 
thelial constituents  of  the 
future  hair-follicle.  The  dif- 
ferentiation of  the  surrounding 
connective  tissue  produces  the 
fibrous  root-sheath,  while 
a  projection  opposite  the  base 
of  the  primitive  epithelial  fol- 
licle contributes  the  tissue  of 

the  hair-papilla.  The  region  immediately  over  the  papilla  is  the 
seat  of  greatest  activity  and  differentiation  :  the  central  cells,  con- 
taining probably  many  eleidin  granules,  become  converted  into  the 
hair  and  its  inner  root-sheath,  while  the  peripheral  cells  of  the 


Section  of  skin  of  fcetal  kitten,  exhibiting  hairs  in 
various  stages  of  development  :  a,  superficial  layer  of 
epidermis  :  b,  stratum  Malpighii  from  which  rudimen- 
tary hair-follicles  extend  into  connective  tissue  (c)  of 
primitive  corium  ;  d,  e,  /,  hairs  in  different  stages  of 
their  development ;  g,  sebaceous  glands  growing  from 
young  hair-follicle. 


280 


NORMAL    HISTOLOGY. 


cylindrical  epithelial  mass  assume  the  character  of  the  external  root- 
sheath.      Subsequent   differentiation    in   the    central    mass  of 
formative  cells  produces  the  individual  layers 
Fig.  315.  0f  the  inner   root-sheath   and  of  the  hair. 

ys  -     ^Hv  • '  $WL '  The  young  hair,  or  lanugo ,  at  first  lies  com- 

pletely embedded  within  the  epidermis, 
its  exit  being  opposed  by  the  cells  occupy- 
ing the  neck  of  the  follicle ;  these  cells 
soften  and  undergo  fatty  degeneration,  when 
the  young  hair  forces  its  way  against  the 
superficial  epithelial  layers.  The  epidermal 
scales  at  first  are  raised,  but  afterwards  they 
are  broken  through  by  the  pointed  ex- 
tremity of  the  growing  hair-shaft.  The 
eruption  of  the  hairs  on  the  head  and  the 
eyebrow  occurs  about  the  close  of  the  fifth 
month  of  foetal  life,  and  is  completed  about 
the  sixth  month  on  the  extremities.  The 
foetal  hairs,  forming  the  downy  covering, 
or  the  lanugo,  never  possess  a  medulla,  and 
are  short-lived,  ceasing  to  grow  towards 
the  end  of  gestation  ;  shortly  after,  or  even 
before,  birth  these  embryonal  hairs  are 
largely  shed  and  replaced  by  more  per- 
manent successors  ;  on  the  face  and  a  few 
other  places,  however,  the  lanugo  remains. 
The  general  renewal  of  the  hairs  after  birth 
corresponds  to  the  periodical  change  of  coat 
so  common  among  the  lower  animals  ;  such 
renewal  is  very  unusual  in  man,  the  replace- 
ment of  the  effete  hairs  continually  taking 
place.  As  soon  as  the  growth  of  a  hair 
is  arrested  the  pressure  induced  by  the 
surrounding  soft  elastic  structures  is  no 
longer  resisted,  and  in  consequence  the 
hair  is  separated  and  lifted  from  its  papilla ; 
such  hairs  possess  knob-like  extremities, 
which  are  lodged  in  corresponding  expan- 
sions of  the  outer  root-sheath.  Beneath 
the  terminal  knob  the  cells  of  this  outer 
root-sheath  grow  out  as  a  new  mass 
towards  the  base  of  the  follicle  ;  from  these 
young  cells  in  due  time  the  new  hair  is  formed,  the  details  of  the 
process  corresponding  with  those  of  the  development  of  the  primary 


[grass  ft'; :  «i»»ai 

mm 


Section  of  hair-follicle  from 
human  scalp,  exhibiting  changes 
accompanying  growth  of  new 
hair:  a,  old  hair,  terminating  in 
expanded  degenerating  end  (a' )  ; 
b,  inner  root-sheath  ending  in 
atrophic  area  at  6' :  c,  outer  root- 
sheath  ;  e,  glassy  membrane;  f, 
lateral  projection  marking  attach- 
ment of  arrector  pili  muscle  (g-,; 
h,  mass  of  new  cells  derived  from 
root-sheath  of  old  follicle  from 
which  formation  of  new  hair  will 
proceed. 


THE   SKIN    AND    ITS    APPENDAGES. 


28l 


hairs.  Coincidently  with  the  growth  of  the  secondary  shaft  the  old 
dead  hair  becomes  shifted  towards  the  surface,  loosened,  and  finally 
entirely  displaced. 

The  development  of  the  sebaceous  glands  starts  as  an  out- 
growth from  the  external  root-sheath  of  the  hair-follicle,  from 
which  knob-like  projections  extend  laterally  ;  these  are  at  first  solid 
flask-shaped  processes,  the  central  cells  of  which  become  filled  with 
fat-particles.  This  fatty  metamorphosis  affects  all  the  cells  occupy- 
ing the  axis  of  the  developing  gland  as  far  as  the  root-sheath  ;  after 
a  time  the  latter  structure  is  penetrated  and  the  degenerated  fatty 
cell-mass  discharged  into  the  hair-follicle  as  the  first  sebaceous 
secretion.  From  the  original  tubular  projection  secondary  com- 
partments are  produced  by  a  repetition  of  the  processes  of  budding 
and  subsequent  hollowing  out  until  the  entire  complement  of  saccules 
has  been  formed.  After  the  disintegration  of  the  central  cells,  the 
peripheral  elements  undergo  similar  change. 

The  development  of  the  sweat-glands  follows  closely  that  of 
the  hairs  and  the  sebaceous  follicles  ;  as  in  these,  so  here,  the  first 
stage  consists  in  the  ingrowth,  during  the  fifth  month,  of  a  solid 
epithelial  club-shaped  process  from  the  stratum  mucosum  into 
the  primitive  corium.  About  the  seventh  month  a  lumen  appears 
within  the  tubular  mass,  an  exit, 

however,    for   some  time  being  FlG-  3l6- 

still  wanting  ;  subsequently  the 
obstructing  epidermal  layers  are 
broken  through.  Somewhat 
before  the  appearance  of  the  lu- 
men the  extremity  of  the  cylin- 
der undergoes  increased  growth, 
resulting  in  the  thickening  and 
convolution  of  the  tube  which 
represents  the  future  coiled 
division  of  the  gland  ;  the  full 
expression  of  the  characteristic 
convoluted  arrangement,  how- 
ever, is  not  attained  until  shortly 
before  birth.  The  muscular 
tissue  of  the  secretory  tubes, 
situated  between  the  basement- 
membrane  and  the  lining  epithe- 
lium, is  present  before  the  close  of  the  ninth  month  ;  its  origin  is 
as  remarkable  as  its  position,  since  the  muscle-cells  are  derived 
from  the  elements  of  the  adjacent  ectoderm.  The  basement-mem- 
brane and  the  fibrous  sheath  are  contributions  from  the  mesoderm. 


&m&g 


&. 


> 

& 


ml  \ 


Section  of  skin  of  human  foetus,  showing  devel- 
oping sweat-glands.  The  latter  grow  as  epithelial 
cylinders  from  the  stratum  Malpighii  of  the  epi- 
dermis into  the  underlying  corium  ;  the  character- 
istic coil  appears  later. 


282 


NORMAL    HISTOLOGY. 


CHAPTER    XVI. 


THE    CENTRAL    NERVOUS    SYSTEM. 


THE   MEMBRANES. 

The  spinal  cord  and  the  brain  are  surrounded  by  their  enveloping 
membranes,  the  dura,  the  arachnoid,  and  the  pia ;  these  afford 
additional  protection  and  support  the  blood-vessels  in  their  course  to 
the  nervous  tissue. 

The  dura  consists  of  interlacing  bundles  of  dense  fibro-elastic 
tissue,  in  the  interspaces  between  which  lie  numerous  plate-like 
connective-tissue  cells  ;  many  irregular  granular  elements  resem- 
bling plasma-cells  occupy  the  more  superficial  layers.  The  narrow 
clefts  between  the  fibrous  bundles  represent  lymph-spaces. 

The  smooth  unattached  surfaces  of  the  dura  are  clothed  with 

a    single    layer   of    endothelial 
Fig.  317.  plates,  while  the  attached  sur- 

faces, on  the  contrary,  are  rough 
and  without  endothelium,  but 
covered  with  fibrous  processes 
for  attachment.  The  inner  sur- 
face of  the  visceral  layer  forms 
the  outer  wall  of  the  subdural 
space,  the  inner  boundary  of 
which  is  contributed  by  the  op- 
posed surface  of  the  arachnoid  ; 
the  two  surfaces,  while  usually  in 
apposition,  are  united  by  very  few 
intervening  bands  of  connective 
tissue.  In  some  places  the  outer 
dural  layer  is  less  intimately  united 
with  the  bone  than  usual,  which 
arrangement  produces  the  epidural  spaces  ;  a  more  or  less  perfect 
endothelial  lining  exists  at  such  points. 

The  dural  layers  vary  in  vascularity  in  different  regions  ;  in  addi- 
tion to  the  intradural  venous  sinuses,  on  either  side  of  the  supe- 
rior longitudinal  sinus  smaller  venous  clefts,  the  parasinoidal  spaces, 
occur  ;  into  these  the  cerebral  veins  directly  open,  the  entrance 
of  the  veins  into  the  sinus  being  thus  indirect.  The  arteries  of  the 
dura  in  many  places  are  surrounded  by  perivascular  lymphatic 


Section  of  membranes  from  brain  of  child  :  D, 
A ,  P,  respectively  the  dura,  the  arachnoid,  and 
the  pia ;  a,  subdural  space ;  i,  meshes  of  sub- 
arachnoidean  space ;  c,  blood-vessel  within  the 
pia  sending  branch  into  cerebral  cortex,  d. 


THE   CENTRAL   NERVOUS   SYSTEM.  28^ 

channels  ;  these  canals  open  into  the  sub-  or  the  epi-dural  spaces 
on  the  one  hand,  and  stand  in  close  relation  with  the  blood-vessels 
on  the  other.  The  veins  of  the  dura  are  of  much  greater  size  than 
the  corresponding  arteries. 

The  nerves  of  the  dura  are  not  numerous,  but  consist  of  both 
medullated  and  pale  fibres,  chiefly  distributed  to  the  walls  of  the 
blood-spaces. 

The  arachnoid  is  a  connective-tissue  membrane  of  great  delicacy, 
the  component  fibres  being  loosely  held  together  rather  than  arranged 
as  distinct  bundles.  The  free  surfaces  of  the  membrane,  including 
the  numerous  trabecular  on  its  inner  side,  are  covered  with  endo- 
thelium. The  arachnoid  lies  closely  applied,  but  slightly  attached, 
to  the  inner  surface  of  the  dura,  while  between  the  arachnoid  and  the 
pia  the  considerable  subarachnoidean  space  exists. 

Scattered  over  the  outer  surface  of  the  arachnoid  small  villous 
elevations  project  into  the  subdural  space ;  a  core  of  connective 
tissue,  covered  by  a  reflection  of  the  endothelium,  constitutes 
these  structures.  In  various  situations,  but  particularly  in  the 
neighborhood  of  the  superior  longitudinal  sinus,  the  arachnoidal 
villi  become  hypertrophied  and  form  the  Pacchionian  bodies  :  these 
press  against  the  opposed  dural  surface  and  push  the  latter  before 
them  where  least  resistant ;  such  points  occur  where  the  lamina  of 
the  dura  separate.  The  arachnoidal  projections  encroach  upon 
the  dura  to  such  a  degree  that  its  tissue  is  largely  absorbed,  the 
cauliflower  excrescence  being  separated  from  the  venous  current  by 
an  extremely  thin  layer.  In  localities  where  the  projections  press 
against  the  bones,  conspicuous  depressions  on  the  inner  cranial  sur- 
face mark  the  positions  of  the  Pacchionian  bodies ;  these  latter  not 
infrequently  contain  small,  hard,  calcareous  concretions,  the 
"  brain-sand."  The  arachnoid  contains  neither  blood-vessels  nor 
nerves. 

The  pia  is  the  vascular  membrane,  and  consists  of  two  lamellae, 
an  outer  layer,  rich  in  blood-vessels,  and  an  inner  stratum,  less 
vascular,  but  closely  associated  with  the  nervous  tissue,  to  which  it 
contributes  a  connective-tissue  framework.  The  pial  stroma  is 
composed  of  interlacing  fibro-elastic  bundles,  between  which  lie 
the  numerous  blood-vessels,  surrounded  by  perivascular  lymphatics ; 
the  vessels,  invested  by  delicate  prolongations  of  both  connective 
tissue  and  lymph-sheath,  pass  into  the  nervous  tissue.  The  free  sur- 
face of  the  pia  is  covered  by  endothelium,  as  are  also  the  trabecular 
subdividing  the  subarachnoidean  space  and  connecting  the  arach- 
noid and  the  pia.  The  pia  of  the  cord  is  composed  of  coarser  fibres 
than  that  of  the  brain. 

The  dura  and  the  arachnoid  do  not  follow  the  irregularities  of 


2g4  NORMAL   HISTOLOGY. 

the  surface  of  the  nervous  masses ;  the  pia  does,  dipping  down  into 
the  fissures  and  penetrating,  as  part  of  the  velum  interpositum,  into 
the  interior  of  the  brain.  The  inner  layer  of  the  pia  is  closely- 
united  to  the  surface  of  the  cord  and  the  brain,  while  the  vascular 
tunic  in  places  is  less  accurately  adapted  :  thus  the  entire  pia  enters 
the  anterior  median  fissure  of  the  spinal  cord,  while  the  inner  layer 
alone  takes  part  in  the  formation  of  the  posterior  median  septum,  or 
' '  fissure. ' ' 

Branched  pigment-cells  are  not  uncommon  in  the  outer  layer 
of  the  pia  ;  these  are  especially  well  developed  on  the  anterior  sur- 
face of  the  medulla,  although  frequently  found  in  other  positions 
along  the  cord  and  at  the  base  of  the  brain.  A  few  non-medullated 
nerve-fibres  have  been  traced  within  the  pia. 

THE   SPINAL    CORD. 

The  spinal  cord,  or  medulla  spinalis,  hangs,  enveloped  by  its 
membranes,  within  the  vertebral  canal,  and  extends  from  the  upper 
border  of  the  atlas,  where  it  becomes  continuous  with  the  medulla 
above,  to  the  lower  border  of  the  first  lumbar  vertebra  below ;  from 
this  level  the  conical  end  of  the  cord,  the  conus  medullaris,  is 
continued  into  the  attenuated  filum  terminale,  the  nervous  matter 
disappearing  about  the  middle  of  this  structure.  While  the  several  di- 
visions of  the  cord  are  distinguished  by  individual  peculiarities,  certain 
general  features  of  arrangement  are  common  throughout  its  length. 

The  cord  is  formed  of  symmetrical  halves  partially  separated 
in  the  mid-line  in  front  by  a  cleft,  the  anterior  median  fissure,  and 
behind  by  an  ingrowth  of  pial  connective  tissue  which  constitutes 
the  posterior  median  fissure,  but  is  really  only  a  fibrous  sep- 
tum. Each  half  of  the  cord  contains  a  crescentic  mass  of  gray 
matter  ;  the  convexities  of  the  crescents  face,  and  are  connected  by 
a  horizontal  bridge,  the  gray  commissure,  the  gray  matter  of  the 
cord  thus  collectively  forming  an  H-like  mass.  The  horns  of  the 
crescents  are  not  equal,  the  anterior  cornua  being  broad  and  robust, 
while  the  posterior  cornua  are  more  slender  and  pointed  and  ex- 
tend almost  to  the  outer  surface. 

The  exterior  of  the  cord  is  closely  invested  by  the  inner  pial 
layer,  from  which  numerous  fibrous  septa  extend  into  the  substance 
of  the  cord,  dividing  the  white  matter  into  certain  pyramidal  areas. 
The  anterior  or  motor  roots  of  the  spinal  nerves  are  formed  by 
bundles  of  fibres  which  escape  from  the  gray  matter ;  these  bundles 
pass  from  the  anterior  cornu  to  the  surface  of  the  cord  associated  in 
groups,  their  exit  being  indicated  by  slight  furrows.  The  position 
at  which  the  posterior  or  sensory  root  appears  on  the  surface,  on 
the  contrary,  is  marked  by  a  distinct  indentation. 


THE  CENTRAL   NERVOUS   SYSTEM.  2§r 

These  furrows  marking  the  anterior  and  the  posterior  roots,  to- 
gether with  the  penetrating  processes  from  the  pia,  divide  the  white 
matter  of  each  half  of  the  cord  into  definite  areas  or  tracts.  The 
anterior  median  fissure  penetrates  a  little  more  than  one-third  of  the 
diameter  of  the  cord,  and  does  not  quite  reach  the  bridge  of  gray- 
substance,  but  leaves  an  intervening  band  which  connects  the  white 
matter  of  the  two  halves  ;  this  constitutes  the  white  commissure  in- 
cluded between  the  gray  bridge  behind  and  the  anterior  median  fissure 
in  front. 

The  part  of  the  cord  embraced  between  the  anterior  median  fis- 


Fig.  318. 


Section  of  spinal  cord  from  cervical  region  of  child  :  a,  anterior  median  fissure  ;  b,  posterior  median 
septum  ;  c,  d,  anterior  and  posterior  horns  of  gray  matter;  e,  /,  anterior  and  posterior  nerve-roots  ; 
f,  lateral  reticulum  of  gray  substance  into  white  matter  ;  h,  median  and  antero-lateral  groups  of 
ganglion-cells  ;  i,  central  canal;  k,  I,  gray  and  white  commissures  ;  A,  L,  P,  anterior,  lateral,  and 
posterior  columns  ;  G,  column  of  Goll ;  B,  column  of  Burdach. 

sure  and  the  anterior  root  is  the  anterior  column ;  the  large  area 
bounded  by  the  anterior  root,  the  gray  matter,  and  the  posterior 
root  forms  the  lateral  column ;  while  the  field  included  between 
the  posterior  root  and  the  posterior  median  septum  corresponds  to 
the  posterior  column.  Since  the  first  two  divisions  are  very 
closely  associated  both  as  to  their  position  and  as  to  their  constit- 
uents, they  are  very  frequently  regarded  as  a  single  column,  the 
antero-lateral.  Each  of  these  principal  segments  is  subdivided 
into  secondary  tracts,  distinguished  by  names  indicating  the  gen- 
eral course  or  the  destination  of  the  component  nerve-fibres. 


286 


NORMAL   HISTOLOGY. 


The  anterior  column  includes  two  tracts  :  the  direct  pyram- 
idal  tract    (Tiirck's  column),   next   the   median  fissure,   and  the 


Fig.  319. 


Diagram  showing  principal  divisions  of  white  matter  of  spinal  cord:  A,  P,  anterior  and 
posterior  horns  of  gray  matter  ;  DP,  direct  pyramidal  tract  ;  GB,  anterior  ground-bundle  ; 
CP,  crossed  pyramidal  tract  ;  DC,  direct  cerebellar  bundle  ;  GT,  Gowers's  or  ascending 
antero-lateral  tract;  DAL,  descending  antero-lateral  patch;  ML,  mixed  lateral  tract; 
BG,  column  of  Burdach  (fasciculus  cuneatus)  and  column  of  Goll  (fasciculus  gracilis). 

anterior  ground-bundle,  or  anterior  radicular  zone,  which  is 
continuous  with  the  adjoining  area  of  the  lateral  region. 

The  lateral  column  contains  a  number  of  secondary  tracts,  two 
of  which  are  especially  prominent,  the  crossed  pyramidal  and  the 
direct  cerebellar.  The  latter  lies  as  a  narrow  zone  at  the  margin 
of  the  cord,  and  extends  from  the  posterior  root  about  half-way  to 
the  anterior  root.  The  crossed  pyramidal  tract  appears  as  an 
oval  area  which  lies  next  the  cerebellar  path  and  in  front  of  the  pos- 
terior root.  The  remainder  of  the  lateral  column  is  occupied  by  a 
number  of  smaller  tracts,  concerning  which  uncertainty  still  exists. 
These  may  be  grouped  into  three  segments  :  an  outer  peripheral, 
the  ascending  antero-lateral  tract,  or  tract  of  Gowers,  a 
middle  area,  the  descending  antero-lateral  tract,  and  an  inner 
zone,  next  the  gray  matter,  the  mixed  lateral  tract,  of  which  the 
posterior  division  contains  probably  sensory  fibres  and  the  anterior 
motor. 

The  posterior  column  is  divided  by  a  fibrous  septum  into  the 
inner  triangular  segment,  the  column  of  Goll,  next  the  median 
septum,  and  an  outer  area,  the  postero-lateral  tract,  or  Burdach's 
column,  lying  between  Goll's  tract  and  the  posterior  horn  and  root. 
Since  the  development  of  the  tracts  above  enumerated  differs  in  the 


THE   CENTRAL   NERVOUS   SYSTEM.  237 

various  regions  of  the  cord,  it  is  evident  that  the  areas  which  they 
present  vary  with  the  plane  of  section  :  the  accompanying  diagram, 
therefore,  indicates  their  relative  positions  rather  than  their  respective 
extent. 

The  framework  of  the  spinal  cord  consists  of  the  penetrating 
pial  processes,  which  divide  the  white  matter  into  numerous  areas 
as  well  as  convey  the  blood-vessels  into  the  nervous  tissue.  From 
the  large  fibrous  partitions  finer  secondary  trabeculae  are  given  off ; 
these,  in  turn,  divide  and  subdivide  until  they  become  lost  as  delicate 
fibrils  among  the  nervous  elements.  In  addition  to  the  framework 
of  connective  tissue  contributed  by  the  pia,  the  specialized  sup- 
porting tissue  of  the  nervous  system,  the  neuroglia,  is  distributed 
throughout  the  cord,  filling  up  the  coarser  meshes  of  the  connective- 
tissue  reticulum  and  intimately  uniting  the  more  important  nervous 
elements. 

The  neuroglia  occurs  immediately  beneath  the  outer  pial  invest- 
ment as  a  condensed  peripheral  zone,  from  which  prolongations 
accompany  the  pial  septa, 
as  well  as  intermingle  with 
the  nerve-fibres  ;  among 
the  latter  lie  the  charac- 
teristic spider-cells, 
sending  their  long,  deli- 
cate processes  between 
the  fibres. 

The  white  matter  of 
the  cord  is  made  up  seem- 
ingly of  great  numbers  of 
small  round  nucleated  ele- 
ments, held  together  by 
the  supporting  neuroglia. 
These  apparent  cells  are 
the  nerve  -  fibres  in 
transverse  section,  the 

supposed  nuclei  being  really  the  cut  axis-cylinders  ;  an  irregularly 
concentric  striation  is  usually  present  around  the  axis-cylinder,  this 
appearance  being  produced  by  the  partial  distortion  of  the  medullary 
substance.  The  nerve-fibres  of  the  cerebro-spinal  axis  possess  no 
neurilemma,  the  surrounding  neuroglia  affording  the  necessary 
protection. 

The  individual  nerve-fibres  composing  the  white  matter  of  the 
cord  vary  greatly  in  diameter  (1-27  fi)  ;  while  the  thick  and  the  thin 
fibres  are  found  side  by  side  in  all  regions  of  the  cord,  certain 
columns  are  characterized   by  the   predominance   of  thick  fibres, 


Portion  of  white  matter  of  human  spinal  cord  :  a,  large 
nerve-fibres  in  section ;  b,  smaller  fibres ;  c,  supporting 
neuroglia ;  d,  spider-cell  ;  e,  connective-tissue  trabecula 
containing  blood-vessel,  f ;  g,  spaces  from  which  sections 
of  nerve-fibres  have  been  lost. 


288 


NORMAL   HISTOLOGY. 


while  other  tracts  contain  mostly  small  ones.  With  reservation,  it 
may  be  assumed  that  motor  fibres  are  generally  the  largest  (15- 
20  (j.):  hence  the  nerves  issuing  from  the  anterior  cornua  contain 
principally  fibres  of  large  size  ;  the  posterior  sensory  nerves  and 
the  sensory  tracts,  on  the  contrary,  contain  chiefly  small  fibres, 
although  a  number  of  fibres  of  large  diameter  are  usually  present. 
The  largest  fibres  occur  within  the  direct  and  crossed  pyramidal 
tracts ;  the  smallest,  within  the  column  of  Goll. 

The  white  commissure  in  man  forms  a  continuous  nervous 
lamella,  .3-5  mm.  in  thickness,  which  separates  the  gray  com- 
missure from  the  bottom  of  the  anterior  median  fissure  ;  in  many 
animals  the  white  commissure  is  incomplete,  being  represented  by 
isolated  commissural  bundles  found  only  at  certain  levels. 

The  gray  matter,  while  presenting  the  general  H-form  through- 
out the  cord,  differs  in  the  details  of  its  arrangement  in  the  several 

regions.  The  gray  mat- 
ter shares  in  the  in- 
creased size  which  char- 
acterizes the  cervical  and 
lumbar  enlargements,  its 
amount  being  absolutely 
as  well  as  relatively 
greatest  in  the  lumbar 
region. 

Typical  cervical  sec- 
tions are  distinguished 
by  their  large  size,  great 
transverse  diameter,  and 
large  H  of  gray  matter, 
the  anterior  cornua  of 
which  are  robust  and 
broad,  while  the  poste- 
rior horns  are  slender. 
Sections  from  the  tho- 
racic region  are  smaller 
than  those  from  either  above  or  below,  and  present  an  almost  circular 
outline ;  the  gray  matter  possesses  crescents  only  slightly  curved,  with 
slender  horns  both  in  front  and  behind.  Cross-sections  of  the 
lumbar  cord  are  recognized  by  being  broad,  having  a  deep  anterior 
fissure,  and  possessing  a  large,  thick  H  which  greatly  encroaches 
upon  the  white  matter.  The  latter  diminishes  relatively,  as  well  as 
absolutely,  on  reaching  the  conus  medullaris,  where  it  is  reduced 
to  a  mere  shell. 

The  gray  matter  of  the  halves  is  united  by  the  gray  commissure, 


Section  of  spinal  cord  from  thoracic  region  of  child  :  V,  D, 
ventral  (anterior)  and  dorsal  (posterior)  median  fissures ;  C, 
column  of  Clarke. 


THE   CENTRAL   NERVOUS   SYSTEM.  28g 

in  the  middle  of  which  lies  the  minute  central  canal,  the  direct 


**g  ?0  a'1      ~^>£M 


Section  of  spinal  cord  from  lower  part  of  lumbar  region  of  child  :    V,  D,  ventral  and  dorsal 
median  fissures. 

continuation  of  the  ventricular  cavities  of  the  encephalon  and  the 
remains  of  the  large  neural  canal  of  early  fcetal  life.     That  part  of 

Fig.  323. 


Portion  of  section  of  spinal  cord  of  calf,  including  central  canal  and 
commissures :  a,  b,  anterior  and  posterior  median  fissures  ;  c,  central  canal, 
lined  with  ciliated  epithelium,  d  ;  e,  surrounding  substantia  gelatinosa  ;  /, 
gray  commissure  ;  g,  white  matter ;  h,  white  commissure  ;  i,  decussating 
nerve-fibres  ;  k,  nerve-fibres  in  section. 

the  gray  commissure  lying  in  front  of  the  central  canal  constitutes 

19 


290 


NORMAL   HISTOLOGY. 


Fig.  324. 


the  anterior  gray  commissure,  while  that  behind  the  canal  is  the 
posterior  gray  commissure. 

The  histological  elements  entering  into  the  composition  of  the 
gray  matter  include  the  nerve-cells,  the  nerve-fibres,  the  sub- 
stantia spongiosa,  the  substantia  gelatinosa,  and  the  blood- 
vessels. 

The  most  conspicuous  elements  of  the  gray  matter  are  the  gan- 
glion-cells.    These  are  especially  numerous  in  the  anterior  and 

posterior  horns,  where 
they  form  almost  con- 
tinuous columns.  The 
motor  cells  are  largest, 
those  found  in  the  anterior 
cornu  being  distinguished 
by  their  great  size  (65-130 
fj.),  as  well  as  by  their 
numerous  branched  pro- 
cesses. The  ganglion- 
cells  of  the  anterior 
cornu  are  disposed  in 
groups,  of  which  there 
may  be  recognized  usually 
a  small  median  group, 
occupying  the  inner  part 
of  the  horn,  but  wanting 
in  the  lumbar  region,  and 
a  conspicuous  large  an- 
terolateral group,  which 
lies  in  the  outer  angle  of 
the  horn  ;  the  posterior 
outer  angle,  where  the  an- 
terior cornu  is  broad  and  well  developed,  contains  often  additionally 
a  postero-lateral  or  an  external  group. 

The  ganglion-cells  of  the  posterior  horn  are  much  smaller 
(15-20  ju)  and  somewhat  irregularly  distributed  to  the  outer  side  of 
the  cornu,  particularly  near  its  base. 

In  certain  regions  of  the  cord,  principally  from  the  eighth  cervi- 
cal to  the  third  lumbar  nerve,  much  less  markedly  in  the  upper 
cervical  and  in  the  sacral  region,  a  distinct  cluster  of  nerve-cells  exists 
at  the  juncture  of  the  posterior  root  and  the  gray  commissure,  which 
marks  the  position  of  the  vesicular  column  of  Clarke.  The  gan- 
glion-cells of  this  tract  vary  in  size  (30-90  fi),  but  possess  an  average 
diameter  of  70  /*,  thus  standing  midway  between  the  largest  motor 
and  the  smallest   sensory  elements  ;    their  processes  are  few,  and, 


Anterior  horn  of  gray  matter  of  human  spinal  cord :  g, 
gray  matter  containing  stellate  ganglion- cells;  w,  white 
matter  penetrated  by  bundles  (r)  of  root-fibres. 


THE  CENTRAL   NERVOUS   SYSTEM. 


29I 


together  with  the  long  axes  of  the  cells,  extend  generally  parallel  to 
the  long  axis  of  the  cord. 

The  lateral  horn  when  well  developed,  as  in  the  intermedio- 
lateral  tract  of  the  tho- 
racic region,  also  con-  Fig.  325. 
tains  groups  of  small, 
frequently  bipolar,  cells 
(20-30  fi)  which  re- 
semble the  isolated  cells 
of  the  posterior  cornu. 

In  addition  to  the 
groups  of  nerve  -  cells 
described,  isolated  clus- 
ters of  "outlying" 
ganglion  -  cells  exist 
beyond  the  gray  sub- 
stance, within  the  white 
matter  of  the  antero- 
lateral and  posterior 
columns. 

The  composition 
of  the  gray  matter  is 
very  intricate,  including 
as  it  does  not  only 
nerve-fibres  of  various 
sizes,  both  medullated 
and  non  -  medullated, 
and  countless  fibrils  of 
varying  thickness  de- 
rived from  the  processes  of  the  ganglion-cells,  but  also  the  universally 
present  substantia  spongiosa,  the  modified  neuroglia  of  the  gray 
matter,  which  contributes  additional  nuclei  and  fibrils  of  its  own. 
The  recent  advances  in  our  knowledge  concerning  the  processes  of 
nerve-cells  have  introduced  new  elements  of  complexity,  for  it  must 
be  remembered  that,  in  addition  to  the  richly-branched  protoplasmic 
processes,  the  axis-cylinders  contribute  numerous  fibrils  both  as  the 
collateral  fibres  and  as  the  net-works  of  fine  terminal  fibres  in 
which  the  axis-cylinder  processes  of  cells  of  the  second  type  end. 

The  relation  between  the  various  nerve-fibres  and  the  cells  of  the 
gray  matter  is  a  question  of  great  difficulty  ;  the  researches  of  Golgi, 
Ramon  y  Cajal,  Kolliker,  and  others  within  the  last  few  years  have 
established  that  the  protoplasmic  processes  probably  neither  anas- 
tomose with  one  another  nor  unite  with  nerve-fibres  ;  likewise, 
that  the  axis-cylinder  processes  of  certain  cells  alone  directly  connect 


Portion  of  anterior  horn  of  gray  matter  of  spinal  cord  of  calf  : 
£-,  multipolar  ganglion-cells  lying  within  pericellular  lymph- 
spaces  (/) ;  r,  r,  bundles  of  nerve-fibres  (_/)  passing  from  gray 
matter  to  form  anterior  roots  ;  ■w,  white  matter  ;  fi,  portions  of 
isolated  processes  of  nerve-cells ;  n,  larger  processes  in  section  ; 
v,  blood-vessel. 


292 


NORMAL   HISTOLOGY. 


with  nerve-fibres,  these  being  principally  the  cells  of  the  anterior 
horn,  which  are  continued  into  the  large  motor  nerves,  and  the 

Fig.  326. 


Diagram  illustrating  the  probable  relations  between  the  cells  and  the  fibres  and  the  principal  tracts 
of  the  spinal  cord;  the  left  half  of  the  figure  exhibits  the  communications  of  the  several  varieties  of 
nerve-cells:  A,  P,  anterior  and  posterior  cornua  of  gray  matter;  PR,  posterior  root-bundles;  DP, 
direct  pyramidal  tract  ;  CP,  crossed  pyramidal  tract ;  DC,  direct  cerebellar  path ;  GB,  Gowers's 
tract ;  a,  motor  cells  passing  directly  into  fibres  of  anterior  roots  of  spinal  nerves ;  b,  various  cells  of 
the  antero-lateral  column,  including  elements  of  Clarke's  column  (b1)  and  of  substantia  Rolandi  ; 
some  give  off  collateral  branches  of  remarkable  size;  c,  commissural  cells;  d,  cells  to  posterior 
column ;  e,  Golgi  cells  of  posterior  horn.  The  right  half  of  the  diagram  shows  the  communications 
established  by  means  of  the  collateral  fibres.     (After  Lenhossik.) 

cells  which  give  off  fibres  passing  into  and  forming  part  of  the 
anterior  and  the  lateral  columns  of  the  cord.  The  cells  of  the 
posterior  horn  in  many  cases  probably  do  not  connect  with  nerve- 
fibres,  but  bear  axis-cylinder  processes,  which  break  up  into  delicate 
fibrils  confined  to  the  gray  matter.  It  has  been  shown,  on  the  other 
hand,  that  the  sensory  fibres,  after  dividing  into  ascending  and 
descending  branches  on  entering  the  white  matter,  give  off  lateral 
twigs,  which  run  at  right  angles  to  the  longitudinal  fibres  ;  on  enter- 
ing the  gray  matter  the  horizontal  branches  divide  into  tufts  of  fibrils 
which  end  free,  often,  however,  in  close  relation,  but  without 
anatomical  continuity,  with  the  nerve-cells.  Such  terminal  sen- 
sory fibres  are  especially  numerous  in  Clarke's  column  and  in  the 
substantia  gelatinosa,  to  the  formation  of  whose  intricate  fibrillar 
complex  they  largely  contribute.     The  accompanying  diagram,  after 


THE   CENTRAL   NERVOUS   SYSTEM.  2Q, 

Lenhossek,  illustrates  the  various  groups  of  nerve-cells  now  recog- 
nized as  taking  part  in  the  composition  of  the  gray  matter,  as  well 
as  the  assumed  communications  established  by  the  collateral  fibres 
within  the  cord. 

In  addition  to  the  nerve-cells  and  the  fibres,  the  gray  matter  is 
everywhere  pervaded  by  the  supporting  and  uniting  substantia 
spongiosa;  this  ground-substance  is  composed  of  neuroglia  and 
branched  connective-tissue  cells,  the  latter  being  rather  more  nu- 
merous than  in  the  white  matter.  Covering  the  posterior  cornu 
and  immediately  surrounding  the  central  canal  of  the  cord,  the 
ground-substance  is  modified  to  become  the  apparently  almost  homo- 
geneous substantia  gelatinosa ;  the  mass  capping  the  posterior 
horn,  the  substantia  gelatinosa  Rolandi,  contains  some  few  fusi- 
form nerve-cells  and  presents  a  striation  produced,  in  part  at  least, 
by  the  course  of  the  posterior  root-fibres.  The  zone  of  clear  ground- 
matrix  surrounding  the  central  canal  of  the  cord,  the  substantia 
gelatinosa  centralis,  very  closely  resembles  that  on  the  posterior 
cornu,  and  may  be  regarded  as  very  similar,  if  not  identical,  in 
nature  ;  in  certain  regions  (cervical  and  dorsal)  the  gelatinous  sub- 
stance encroaches  somewhat  upon  the  gray  commissure. 

The  central  canal  occupies  the  gray  commissure,  being  contin- 
uous with  the  cavity  of  the  fourth  ventricle  above,  and  ending  blindly 
below  in  the  upper  half  of  the  filum  terminale.  The  canal  does  not 
occupy  the  centre  of  the  gray  commissure,  since  it  lies  rather  ven- 
trally  to  that  point,  especially  in  the  lower  part  of  the  cord. 

The  columnar  epithelium  lining  the  canal  is  an  extension  of 
that  of  the  cerebral  ventricles.  In  children,  and  in  many  animals 
at  all  ages,  the  surface  of  the  cells  directed  towards  the  lumen  is 
clothed  with  cilia ;  the  opposite  ends  of  the  cells  terminate  in  long 
slender  processes,  which  extend  deeply  into  the  surrounding  struct- 
ures. The  lining  cells  represent  the  spongioblasts,  which  in  the 
embryonic  cord  closely  crowd  around  the  central  canal  and  send  long 
delicate  fibres  from  their  outer  ends  through  the  cord  as  far  as  the 
pia,  while  from  their  inner  surface  the  hair-processes,  the  cilia,  pro- 
ject into  the  central  canal.  The  epithelium,  with  the  subjacent 
neuroglia  layer  on  which  the  cells  rest,  constitutes  the  ependyma. 

The  form  and  size  of  the  central  canal,  which  represents  the 
remains  of  the  primitive  neural  tube,  vary  in  the  different  divisions  : 
in  the  upper  cervical  region  its  cross-section  is  somewhat  quad- 
rilateral, from  the  level  of  the  fifth  cervical  nerve  becoming  oval 
or  slit-like,  with  the  cleft  placed  parallel  with  the  commissure.  In 
the  dorsal  region  the  canal  gradually  approaches  the  circular  form, 
while  in  the  lumbar  it  once  more  becomes  a  compressed  oval, 
with,  however,  the  long  diameter  coinciding  with  the  sagittal  plane. 


294 


NORMAL   HISTOLOGY. 


The  canal  of  the  sacral  cord  and  of  the  conus  medullaris  assumes  a 
1-form,  consisting  of  a  ventral  wider  arm  and  a  narrow  dorsal  ex- 
tension ;  an  irregular  dilatation  in  the  lower  part  of  the  conus  has 
received  the  name  ventriculus  terminalis.  The  cords  of  chil- 
dren and  of  many  animals  contain  a  completely  pervious  central 
canal ;  in  the  human  cord  in  later  life  this  is  usually  more  or  less 
occluded,  although  much  variation  exists  in  this  respect.  The  upper 
cervical,  lower  lumbar,  and  sacral  regions  usually  contain,  even  in 
the  adult,  a  partially  pervious  tube.  Overgrowth  of  the  lining 
cells,  as  well  as  of  the  subepithelial  substantia  gelatinosa,  is  the  prin- 

Fig.  327. 


Section  of  spinal  cord  of  human  embryo  stained  by  Golgi  silver  method  ;  the  left  half  of  the  figure 
exhibits  the  neuroglia-cells,  while  the  right  shows  the  elements  constituting  the  framework  of  the  epen- 
dyma.     (After  Lenhossik.) 


cipal  factor  in  the  closure  of  the  central  canal,  which,  however,  must 
be  regarded  as  a  normal  change  and  not  a  pathological  process. 

The  blood-vessels  of  the  substance  of  the  cord  are  arranged  in 
two  groups :  those  entering  at  the  periphery,  including  the 
larger  branches  which  follow  the  connective-tissue  septa  ;  those  de- 
rived from  the  arteria  sulci,  given  off  from  the  anterior  spinal 
artery  and  lodged  within  the  anterior  median  fissure,  from  which 
branches  are  distributed  to  the  gray  matter.  Of  the  numerous 
arteries  which  enter  at  the  circumference,  the  finer  usually  terminate 


THE   CENTRAL   NERVOUS   SYSTEM. 


295 


within  the  white  substance,  while  the  coarser  alone  penetrate 
into  the  gray  matter,  the  outer  zone  of  which  they  in  part  supply. 
The  vessel  occupying-  the  posterior  median  septum,  the  arteria  fis- 
surae  posterioris,  is  the  most  important  of  the  peripheral  branches  : 
twigs  also  accompany  the  anterior  and  posterior  root-bundles. 

At  the  bottom  of  the  anterior  median  fissure  the  arteria  sulci 
divides  into  two  sulco-commissural  branches,  which,  diverging 
slightly,  enter  the  gray  matter  to  the  inner  side  of  the  base  of  the 
anterior  horn.  After  a  short  course  within  the  gray  substance,  these 
vessels  break  up  into  a  number  of  twigs,  which  soon  form  close  cap- 
illary net-works  within  the  anterior  and  middle  parts  of  the  gray 

Fig.  328. 


Section  of  injected  spinal  cord  of  child  :  s,  sulcal  branch  of  anterior  spinal  artery  occupying  anterior 
median  fissure  ;  c,  c,  sulco-commissural  vessels  from  sulcal  artery  passing  to  gray  matter  to  form 
dense  net-work  ;  /,  posterior  spinal  artery,  sending  off  twigs  to  white  matter  ;  around  margin  of  cord 
numerous  peripheral  vessels  enter  white  substance  to  form  open  net-work. 


crescents  ;  a  branch  of  some  size  passes  backward  to  supply  the 
region  corresponding  to  Clarke's  column.  The  sulco-commissural 
artery  likewise  gives  off  vertical  anastomosing  branches,  one 
passing  brainward,  the  other  caudalward,  to  unite  with  similar  off- 
shoots from  the  corresponding  arteries  of  different  planes.  The  veins 
follow  in  general  the  course  of  the  corresponding  arteries  :  some  of 
the  blood,  however,  brought  by  the  sulcal  artery  is  carried  off  by  the 
peripheral  veins. 

THE   MEDULLA. 

The  differences   between  the   medulla  and   the   spinal   cord   are 
rather  of  arrangement  than  of  any  great  variation  in  structural 


2Q5  NORMAL   HISTOLOGY. 

elements,  since  the  tissues  of  the  cord  are  prolonged  into  the  medulla, 
where  the  increased  importance  of  parts  before  relatively  incon- 
spicuous, together  with  the  addition  of  new  masses  of  nervous  matter, 
brings  about  the  redisposition  of  the  structures  continued  from 
the  cord.  The  changes  taking  place  in  the  transition  of  the  cord 
into  the  medulla  consist  primarily  in  a  modification  of  the  gray 
matter ;  the  principal  factors  are  the  gradual  increase  in  the  size 
of  the  tracts  of  the  posterior  column  and  the  decussation  of  fibres 
from  the  lateral  column  destined  to  aid  in  forming  the  anterior  pyra- 
mids. The  changes  wrought  by  the  first 
Fig.  329.  factor  are  earliest  indicated,  and  affect  par- 

ticularly the  posterior  cornua  of  the  gray 
substance,  while  the  second  modifies  the 
anterior  horns. 

An  intimation  of  the  changes  to  follow 
is  seen  in  sections  as  low  as  the  first,  or 
even  second,  cervical  nerve  in  the  thick- 
ened club-shaped  accumulation  of  gray 
matter  representing  the  posterior  cornu, 
connected  by  an  extended  and  attenuated 
stalk  with  the  chief  mass.  With  the  pro- 
gressive increase  in  the  size  of  the  columns 
of  Goll  ( funiculus  gracilis)  and  the  col- 
umns of  Burdach  {funiculus  cuneatus) 
the  posterior  horns  are  displaced  more  and 
more  laterally  and  ventrally  until  the  cornua  with  their  supporting 
necks  lie  nearly  horizontally,  forming  almost  a  right  angle  with  the 
posterior  median  septum.  The  increased  gray  mass  of  the  horns — 
the  caput  cornu — not  only  reaches  the  surface,  but  gradually  dis- 
plays its  growth  by  the  formation  of  the  projection  known  as  the 
funiculus  of  Rolando,  which,  higher  up,  expands  into  the  tubercle 
of  Rolando.  The  greater  size  which  the  tracts  of  the  posterior 
column  assume  is  produced  not  only  by  increased  component  nerve- 
fibres,  but  also  by  the  accession  of  masses  of  gray  matter,  the  nu- 
cleus gracilis  and  the  nucleus  cuneatus,  derived  as  extensions  of 
the  thickened  base  of  the  posterior  horn.  These  gray  nuclei  are 
at  first  narrow,  but  become  more  robust  as  the  medulla  is  ascended, 
until  they  present  the  conspicuous  masses  producing  externally  the 
elevations  of  the  clavus  and  the  cuneate  tubercle.  These  nuclei 
are  covered  over  by  a  thin  sheet  of  white  matter.  Embedded  within 
the  latter,  external  to  the  nucleus  cuneatus,  lies  the  small  accessory 
or  external  cuneate  nucleus. 

With  the  opening  out  of  the  central  canal  of  the  cord  into  the 
fourth  ventricle  the  gray  matter  lying  originally  dorsally  to  the  canal 


Diagram  of  spinal  cord  indicating 
the  paths  taken  by  fibres  of  crossed 
pyramidal  tract  (6)  to  gain  the  an- 
terior columns  (a),  and  by  fibres  of 
posterior  column  (s)  higher  up  to 
form  sensory  decussation.  (After 
Testut.) 


THE   CENTRAL   NERVOUS   SYSTEM.  2Q7 

becomes  laterally  displaced,  while  the  remains  of  the  base  of  the 
anterior  horn  come  to  the  surface  of  the  ventricular  floor,  and,  in- 
creasing in  size,  form  the  projection  of  the  funiculus  teres.  A 
longitudinal  column  of  large  nerve-cells  occupies  part  of  this,  forming 
the  nucleus  from  which  the  numerous  bundles  of  the  roots  of  the 
hypoglossal  nerve  arise. 

The  changes  affecting  the  anterior  cornua  of  the  cord  are  produced 
primarily  by  the  decussation  of  those  fibres  of  the  lateral  column 


Diagram  of  lower  end  of  medulla  at  level  of 
decussation  of  anterior  pyramids  :  a,  anterior 
pyramidal  tracts  ;  b,  posterior  median  septum  ; 
c,  fibres  of  crossed  pyramidal  tracts  crossing 
(d)  to  anterior  pyramid  of  opposite  side ;  e, 
anterior  horn  of  gray  matter  isolated  by  decus- 
sating fibres ;  f,  remains  of  bases  of  anterior 
horns;  g,  nucleus  gracilis;  h,  enlarged  and 
displaced  posterior  horns  of  gray  matter. 
(After  Testut-Duval.) 


Diagram  of  medulla  through  lower  part  of  olivary 
body  :  a,  anterior  pyramidal  tract ;  b,  posterior 
median  groove ;  c,  gray  matter  representing  bases 
of  anterior  cornua,  the  latter  lying  isolated  at  e, 
forming  nucleus  lateralis ;  d,  decussating  fibres  of 
formatio  reticularis ;  g,  nucleus  gracilis ;  h,  gray 
matter  of  bases  of  posterior  horns ;  k,  nucleus 
cuneatus  ;  i,  remains  of  posterior  horns,  substantia 
gelatinosa  of  Rolando ;  j,  ascending  root  of  tri- 
facial nerve ;  tu,  pneumogastric,  n,  hypoglossal, 
nerve ;  o,  nucleus  dentatus  of  olive ;  /,  mesial 
accessory  olive ;  s,  sensory  portion  of  anterior 
pyramids.     (After  Testut-Duval.) 


which  contribute  to  the  formation  of  the  anterior  pyramids.  The 
fibres  of  the  crossed  pyramidal  tract,  in  taking  the  shortest  course 
to  reach  the  point  of  decussation,  cut  obliquely  through  the  gray 
substance  in  such  a  manner  that  the  anterior  cornu  becomes  broken 
up,  its  caput  being  entirely  separated  ;  the  remaining  portion  of 
its  base  forms  a  small  mass  of  gray  matter  lying  ventro-laterally  to 
the  central  canal.  The  isolated  segment  of  the  anterior  cornu  is 
pushed  to  the  side  by  the  development  of  the  pyramid,  and,  higher 
up,  by  the  additional  displacement  caused  by  the  appearance  of  the 
olivary  body  between  the  caput  cornu  and  the  pyramid  ;  in  conse- 
quence the  separated  part  is  displaced  both  laterally  and  dor- 
sally,  and  becomes  the  lateral  nucleus,  taking  up  a  position  in 
close  relation  with  the  now  ventrally  situated  posterior  horn.  By 
the  penetration  of  transverse  and  longitudinal  fibres  the  greater  part 


2Q3  NORMAL   HISTOLOGY. 

of  the  separated  area  is  broken  up  into  a  coarse  net-work  of  gray 
matter  containing  nerve-cells  and  intersecting  fibres — the  formatio 
reticularis. 

The  transverse  or  deep  arcuate  fibres,  which  take  part  in  the 
formatio  reticularis,  from  the  mid-line  curve  outward  and  backward 
towards  the  funiculus  gracilis,  the  funiculus  cuneatus,  the  olivary- 
body,  and,  higher  up,  the  corpus  restiforme. 

Above  the  level  of  the  decussation  of  the  pyramids,  in  suitable 
sections,   fibres  from   the   nucleus   of  the  funiculus  cuneatus,    and 

Fig-  333- 


Section  of  medulla  at  level  nf  sensory 
decussation  :  a,  anterior  pyramidal 
tracts  ;  i,  posterior  median  septum  ; 
c,  h,  gray  matter  representing  bases  of 
anterior  and  posterior  cornua  ;  e,  iso- 
lated anterior  horns  ;  f,  bundles  of  sen- 
sory fibres  displacing  posterior  horn  ; 
g,  nucleus  gracilis  ;  i,  posterior  horn, 
substantia  gelatinosa  of  Rolando ;  k, 
nucleus  cuneatus ;  /,  decussating  sen- 
sory fibres  crossing  (d)  to  opposite 
anterior  pyramids ;  nt,  root-fibres  of 
hypoglossal  nerve.  (After  Testut- 
Duval.) 


Diagram  of  medulla  through  olivary  bodies  :  a, 
anterior  pyramidal  tracts  ;  b,  floor  of  fourth  ven- 
tricle;  c,  remains  of  gray  matter  of  base  of  anterior 
horns,  nucleus  of  hypoglossal  nerve  ;  c' ',  accessory 
hypoglossal  nucleus  ;  d,  decussating  fibres  of  for- 
matio reticularis ;  e,  nucleus  ambiguus ;  g,  gray 
matter  of  posterior  funiculus,  including  h,  which 
represents  base  of  posterior  horn ;  i,  substantia 
gelatinosa  of  Rolando  ;  j,  ascending  root  of  tri- 
facial nerve ;  h,  restiform  nucleus ;  /,  funiculus 
solitarius  ;  m,  root-fibres  of  pneumogastric  nerve  ; 
n,  hypoglossal  nerve;  o,  nucleus  dentatus  of  olive; 
/,  g,  dorsal  and  mesial  accessory  olivary  nuclei ; 
r,  external  arcuate  fibres  ;  s,  sensory  portion  of 
anterior  pyramid.     (After  Testut- Duval.) 


probably  from  that  of  the  funiculus  gracilis  as  well,  are  seen  passing 
obliquely  and  ventrally  to  cross  to  the  opposite  side,  there  becoming 
continuous  with  tracts  proceeding  to  higher  parts  of  the  brain.  These 
crossed  fibres  constitute  the  superior  or  sensory  decussation,  an 
arrangement  especially  well  displayed  in  the  partially  medullated  tracts 
of  the  foetal  medulla.  The  longitudinal  fibres  of  the  substantia 
reticularis  are  principally  contributed  by  the  antero-lateral  tracts  of 
the  cord,  those  from  the  anterior  column  occupying  the  median  area, 
those  from  the  lateral  column  lying  more  to  the  side. 

The  median  raphe,  in  addition,  contains  fibres  extending  dorso- 
ventrally,  which  emerge  at  the  anterior  median  fissure  to  become 
continuous  with  the   superficial    arcuate   fibres,  encircling  with 


THE   CENTRAL   NERVOUS   SYSTEM. 


299 


their  loops  the  anterior  pyramids  and  the  olivary  bodies.  The 
nerve-cells  are  not  uniformly  distributed  within  the  gray  matter 
of  the  reticulum,  since  in  the  ventral  portion  of  the  medulla 
the  cells  are  very  sparingly  distributed,  or  even  wanting,  while 
in  the  lateral  area,  where  the  remains  of  the  anterior  cornu  are 
found,  the  nerve-cells  are  numerous  and  of  large  size. 

On  reaching  the  level  of  the  olivary  bodies,  new  groups  of  ele- 
ments are  introduced ;  of  these  the  most  important  is  the  nucleus 
of  the  olive,  or  corpus  dentatum.  This  consists  of  a  wavy  band 
of  gray  matter  so  disposed  that  it  forms  collectively  a  compressed 
ovoid  capsule  or  shell,  closed  externally,  but  open  towards  the 
median  side,  through  which  hilus  the  nerve-fibres  gain  access  to 

Fig.  334. 


fb^M^0^^m'm 


9- 

Section  of  medulla  of  child  through  olivary  bodies  :  a,  anterior  median  groove ;  b,  raphe  ;  c, 
formatio  reticularis  ;  d,  gray  matter  of  nucleus  dentatus  of  olive  ;  e,  dorsal  accessory  olivary  body  ; 
f,  root-fibres  of  hypoglossal  nerve  ;  g,  nucleus  arciformis ;  h,  external  arcuate  fibres  ;  i,  anterior 
pyramidal  tract ;  k,  remains  of  nucleus  lateralis  ;  /,  substantia  gelatinosa  of  Rolando  and  fibres  of 
ascending  trifacial  root  ;  m,  n,  gray  matter  of  posterior  funiculus  ;  o,  funiculus  solitarius  ;  fi,  nucleus 
ambiguus  ;  g,  root-fibres  of  pneumogastric  nerve  ;  r,  s,  hypoglossal  and  vagus  nuclei ;  /,  nerve-cells 
of  posterior  funiculus  ;  u,  posterior  medullary  velum  closing  in  fourth  ventricle,  IV. 


the  interior  of  the  nucleus.  The  wavy  zone  of  gray  matter  is 
composed  of  neuroglia,  in  which  lie  numerous  small  multipolar 
ganglion-cells. 

Two  additional  small  areas  of  gray  substance  are  seen  in 


,00  NORMAL   HISTOLOGY. 

close  proximity  to  the  corpus  dentatum  :  these  are  the  dorsal  or 
outer  and  the  mesial  or  inner  accessory  olivary  nuclei,  the 
first  of  which  lies  behind  the  olivary  nucleus,  near  and  parallel  to 
its  wavy  band,  while  the  second  lies  almost  across  the  open  end  of 
the  corpus  dentatum. 

Attention  has  already  been  directed  to  the  tract  of  large  nerve- 
cells  which  lies  near  the  median  line  and  represents  the  nucleus 
of  the  hypoglossal  nerve.  In  the  lower  part  of  the  medulla, 
before  the  central  canal  opens  out  into  the  ventricle,  a  group  of 
numerous  smaller  cells  lies  close  but  dorsally  to  the  nucleus  just 
mentioned ;  as  the  central  canal  approaches  the  surface  the  tissues 
forming  its  former  dorsal  border  become  gradually  laterally  displaced, 
in  consequence  of  which  this  group  of  nerve-cells  then  comes  to  lie 
outside  of  the  hypoglossal  nucleus.  These  cells  form  a  continuous 
column  throughout  almost  the  length  of  the  medulla,  constituting 
a  common  nucleus  of  the  spinal  accessory,  pneumogastric,  and 
glosso-pharyngeal  nerves. 

The  four  principal  tracts  of  the  medulla  are  made  up  chiefly 
of  the  continuations  of  the  columns  of  the  cord  ;  without  entering 
into  a  detailed  account  of  these  structures,  a  brief  outline  of  the 
most  important  of  the  constituents  of  the  tracts  may  here  find 
place. 

i.  The  anterior  pyramid  is  composed  of  two  sets  of  fibres  :  the 
continuation  of  the  direct  pyramidal  tract  of  the  anterior  column  of 
the  cord,  which  does  not  take  part  in  the  decussation  of  the  pyra- 
mids, and  the  continuation  of  the  crossed  pyramidal  tract  of  the 
lateral  column.  After  its  formation  in  this  manner  the  anterior 
pyramid  becomes  divided  into  three  very  unequal  groups  :  (a)  the 
majority  of  its  fibres  continue  directly  into  and  through  the  pons 
to  the  cerebrum  ;  (3)  certain  fibres  pass  beneath  the  olive,  join- 
ing fibres  from  the  latter  to  aid  in  forming  the  fillet;  (c)  a  few 
fibres  are  deflected  to  the  restiform  body  and  pass  probably  to  the 
cerebellum. 

2.  The  lateral  tract  claims  all  the  fibres  of  the  lateral  column  not 
included  in  the  crossed  pyramidal  and  the  direct  cerebellar  tract, 
together  with  the  external  anterior  or  ground-bundle,  since  the  latter 
really  is  a  part  of  the  adjacent  tract  of  the  lateral  column.  The 
antero-lateral  fibres  enter  beneath  and  at  the  side  of  the  anterior 
pyramid  and  pass  under  the  olivary  body  and  the  arcuate  fibres  to 
take  part  in  making  up  the  formatio  reticularis ;  as  the  fasciculus  teres 
they  appear  in  the  floor  of  the  fourth  ventricle,  and  later  in  the  teg- 
mentum of  the  crus. 

3.  The  restiform  body  contains  constituents  from  a  number  of 
sources  ;  these  may  be  arranged  in  two  groups, — those  derived  from 


THE   CENTRAL   NERVOUS   SYSTEM.  3OI 

the  cord  and  those  arising  from  the  medulla.  The  first  group 
comprises  : 

(a)  The  direct  upward  continuation  of  the  postero-external  (Bur- 
dach's)  column. 

(F)  The  direct  cerebellar  tract  of  the  lateral  column. 

(c)  The  fibres  of  the  postero-median  (Goll's)  tract  of  the  posterior 
column  ;  this  latter  is  prolonged  into  the  posterior  pyramid,  which, 
in  turn,  is  absorbed  by  the  restiform  body. 

Those  arising  within  the  medulla  are  : 

(d)  The  fibres  of  the  funiculus  Rolandi. 

(e)  The  fibres  contributed  by  the  anterior  pyramid. 

(f)  Some  arcuate  fibres  issuing  from  the  anterior  median  fissure. 
4.  The  posterior  pyramid  is  the  upward  prolongation  of  the 

postero-median  column  of  the  cord.  On  approaching  the  lower 
angle  of  the  fourth  ventricle,  this  column,  or  the  funiculus  gracilis, 
exhibits  the  pronounced  thickening  of  the  clavus  with  its  contained 
nucleus,  and  then,  diverging  from  its  fellow  of  the  opposite  side,  tapers 
into  the  restiform  body. 

THE    PONS. 

The  pons,  as  may  be  inferred  from  the  mutual  relations  of  the  sev- 
eral divisions  of  the  brain  which  it  connects,  consists  very  largely  of 
bundles  of  nerve-fibres  ;  in  addition  to  these,  areas  of  gray  mat- 
ter, the  pontine  nuclei,  supplement  the  nerve-fibres  in  making  up 
its  mass.  On  section  the  pons  exhibits  two  portions,  the  dorsal  and 
the  ventral.  The  latter  contains  the  principal  commissural  tracts 
connecting  the  hemispheres  of  the  cerebellum,  and  constitutes  a 
robust  mass  of  transverse  fibres  ;  through  this  the  longitudinal 
bundles  of  the  anterior  pyramids  of  the  medulla  force  their  way 
in  their  course  to  the  cerebrum.  In  the  lower  half  of  the  pons  the 
pyramidal  fibres  are  collected  into  two  closely-packed  groups  of 
bundles,  one  on  either  side  of  the  mid-line,  which  are  enveloped  in 
front  and  behind  by  a  layer  of  transverse  fibres;  higher  up,  above 
the  middle  of  the  pons,  the  pyramidal  tracts  become  separated  by  the 
penetrating  transverse  bundles  into  a  number  of  fasciculi.  Among 
the  transverse  tracts,  therefore,  are  recognized  the  ventral  or  super- 
ficial bundles,  the  dorsal  or  deep  bundles,  and  the  middle  or 
penetrating  bundles.  Small  multipolar  cells  are  found  widely 
distributed  in  the  ventral  region  of  the  pons  within  the  gray  matter 
which  occupies  the  interfibrillar  interstices. 

The  dorsal  portion  of  the  pons  consists  largely  of  structures  rep- 
resenting the  continuation  of  parts  already  encountered  below,  espe- 
cially of  the  formatio  reticularis  and  of  the  dorsal  tracts  of  gray 
substance.     In  addition  to  the  gray  matter  scattered  throughout 


302 


NORMAL   HISTOLOGY. 


the  reticulum,  other  localizations  represent  important  nuclei  of  cranial 
nerves.  The  sheet  of  gray  matter  lying  in  the  lower  half  of  the  ven- 
tricular floor  is  continued  over  the  pons,  and  there  gives  rise  to 
nuclei  connected  with  the  V,  VI,  VII,  and  VIII  nerves.  While  the 
details  of  the  sections  must  vary  with  each  plane,  the  general  dis- 
position of  the  structures  is  shown  in  sections  passing  through  at 
about  the  middle  of  the  fourth  ventricle.     In  such  sections  the  dorsal 


Fig.  335. 


Section  through  upper  part  of  human  pons:  1,  fourth  ventricle  ;  2,  valve  of  Vieussens  lined  with 
ependyma  ;  2',  white  matter  of  anterior  medullary  velum  ;  2",  gray  matter  of  lingula  ;  3,  descending 
root  of  trifacial  nerve  ;  4,  substantia  ferruginea  ;  5,  posterior  longitudinal  bundle  ;  6,  formatio  reticu- 
laris ;  7,  groove  indicating  boundary  between  tegmentum  and  ventral  part  of  pons  ;  8,  superior  cere- 
bellar peduncle;  9,  mesial  fillet;  9',  lateral  fillet;  10,  transverse  fibres  of  pons;  11,  longitudinal 
fibres;   12,  raphe;  V,  trifacial  nerve.     (After  Testut-Stilling.) 

or  tegmental  portion  of  the  pons  bears  a  resemblance  to  the  me- 
dulla, the  gray  dorsal  stratum  giving  rise  to  fibres  which  pierce  the 
reticulum  in  their  course  to  the  free  surface. 

At  a  somewhat  higher  level,  lateral  groups  of  pigmented  nerve- 
cells  occupy  the  floor  of  the  fourth  ventricle  ;  these  cells  are  so 
dark  that  they  collectively  present  an  area  visible  to  the  unaided  eye, 
the  substantia  ferruginea  ;  seen  through  the  stratum  of  white 
fibres  forming  the  immediate  floor  of  the  ventricle,  this  area  appears 
of  a  bluish-gray  or  slate-color  and  constitutes  the  locus  cceru- 
leus.  Close  to  this  pigmented  area,  lying  to  its  mesial  side  and  near 
the  raphe,  an  angular  tract,  known  as  the  posterior  longitudinal 
bundle,  extends  beneath  the  gray  matter  of  the  ventricle,  just  at  the 
dorsal  border  of  the  reticular  formation.     This  fasciculus,  also  prom- 


THE   CENTRAL    NERVOUS    SYSTEM. 


303 


inent  at  higher  levels,  is  the  continuation  of  fibres  from  the  anterior 
ground-bundle  of  the  cord. 


THE   CRURA. 


The  crura  cerebri,  or  cerebral  peduncles,  resemble  the  pons 
in  general  arrangement,  since  they  consist  of  a  ventral  portion,  the 
crusta  pedunculi,  or  the  cerebral  peduncle  proper,  made  up 


Fig.  336. 


Section  through  human  cerebral  peduncles  at  point  of  emergence  of 
oculo-motor  nerve  :  C,  crusta,  separated  from  tegmentum  ( Tg)  by  sub- 
stantia nigra  (S)  ;  R,  raphe  dividing  formatio  reticularis  ;  F,  longitudinal 
bundles  of  latter  ;  O,  groups  of  nerve-cells  connected  with  origin  of  oculo- 
motor fibres  (,0m)  ;  Tf,  cells  connected  with  origin  of  trifacial  nerve;  A, 
aqueduct  of  Sylvius;  CQ,  anterior  corpora  quadrigemina.    (After  Krause.) 

exclusively  of  ascending  and  descending  fibre-tracts,  and  of  a  dorsal 
portion,  the  tegmentum,  which  contains  the  prolongation  of  the 
formatio  reticularis  and  of  the  dorsal  stratum  of  the  gray  substance 


<iqa  NORMAL   HISTOLOGY. 

of  the  medulla  and  the  pons.  On  transverse  section  of  the  crura, 
it  is  seen  that  the  tegmental  halves  are  united,  while  the  two 
peduncular  portions  are  widely  separated  and  are  attached  to 
the  tegmentum  alone  ;  the  oblique  line  of  this  juncture  is  indicated 
within  the  section  by  a  deeply  pigmented  area,  the  substantia 
nigra. 

The  crusta  is  hemi-cylindrical  in  section,  but  the  encroachment 
of  the  substantia  nigra  reduces  the  area  devoted  to  the  ascending  and 
descending  fibres  to  a  narrow  crescent,  whose  convexity  corre- 
sponds to  the  external  outline  of  the  peduncle,  while  the  concavity 
embraces  the  dark  field.  Since  the  tracts  of  the  ascending  fibres  of 
the  peduncle  greatly  exceed  the  pyramidal  bundles  of  the  pons,  it  is 
evident  that  many  additional  fibres  have  arisen  within  the  peduncles. 
On  reaching  the  cerebral  hemispheres  in  their  course  upward,  the 
tracts  of  the  crusta  become  continuous  with  the  fibres  constituting 
the  internal  capsule. 

The  substantia  nigra,  separating  the  crusta  and  the  tegmentum, 
forms  a  tract  of  gray  matter  extending  from  the  upper  border  of  the 
pons  forward  as  far  as  the  mammillary  bodies  ;  while  it  gradually 
diminishes  in  its  forward  course,  the  mesial  edge  of  the  mass  becomes 
thickened  in  the  vicinity  of  the  oculo-motor  groove.  The  area  owes 
its  exceptional  color  to  irregular  groups  of  deeply  pigmented 
multipolar  cells  embedded  within  a  finely  granular  ground-sub- 
stance. 

The  tegmentum  forms  only  part  of  the  great  nuclear  tract 
continued  through  the  dorsal  portion  of  the  oblongata,  the  pons,  and 
the  peduncle  into  the  subthalamic  region  ;  as  in  the  other  localities, 
so  here,  the  stratum  of  gray  matter  lying  beneath  the  floor  of  the 
neural  tube  and  the  formatio  reticularis  are  its  principal  constit- 
uents. In  addition  to  the  gray  matter  distributed  throughout  the 
reticulum,  groups  of  nerve-cells  are  situated  along  the  floor  of  the 
Sylvian  aqueduct ;  some  of  these  are  of  importance  as  the  nuclei 
of  the  bundles  of  the  oculo-motor  and  the  pathetic  nerve.  Near  the 
middle  of  the  formatio  reticularis,  on  either  side  of  the  raphe,  lies  a 
conspicuous  group  of  large  pigmented  nerve-cells,  the  tegmental  or 
red  nucleus,  so  called  on  account  of  its  brown  or  reddish  hue.  The 
formatio  reticularis  of  the  tegmentum  differs  little  from  the  similar 
structure  at  lower  levels.  In  general,  the  fibres  contained  within 
the  crusta  pass  to  the  striatum  and  to  the  cerebral  cortex, 
while  those  of  the  tegmentum  usually  terminate  in  or  about  the 
thalamus. 

THE   CEREBELLUM. 

The  cerebellum  consists  of  a  peripheral  or  cortical  layer  of  gray 
substance  which  encloses  the  various  tracts  of  nerve-fibres  composing 


THE    CENTRAL    NERVOUS    SYSTEM. 


3o: 


the  white  matter  of  the  medulla,  together  with  certain  additional  gray 
nuclei  embedded  within  the  latter.  On  section,  each  leaflet  of  the 
cerebellum  is  seen  to  be  made  up  of  (i)  a  central  core  of  white 
medullary  substance,  which  blends  into  (2;  the  granule  layer, 
characterized  by  its  "rust-color,"   external  to  which  follows  (3)  the 


Fig.  337. 


Section  of  human  cerebellum,  slightly  magnified  to  show  general  arrangement :  w,  white  matter  of 
medulla  ;  g,  o,  granule  and  molecular  or  outer  layer,  between  which  lies  layer  of  Purkinje's  cells  (p). 

outer  or  molecular  stratum  ;  between  the  latter  and  the  granule 
layer  lies  (4)  the  single  row  of  ganglion-cells  which  constitutes  the 
layer  of  the  cells  of  Purkinje. 

The  granule  layer  forms  a  zone  conspicuous  on  account  of  the 
great  number  of  small  deeply-staining  cells  which  it  contains.  It 
varies  in  thickness,  being  broadest  at  the  summit  of  the  laminae 
and  narrowest  at  the  bottom  of  the  fissures.  Towards  the  outer 
layer  the  zone  is  sharply  defined,  but  it  fades  away  on  the  median 
side  into  the  medullary  substance. 

The  nerve-cells  of  the  granule  layer  are  of  two  kinds, — the 
small  and  the  large  ganglion-cells.  The  former  are  small  (6-7  ,u) 
round  elements,  stain  deeply,  but  possess  so  little  protoplasm  that 
the  greater  part  of  the  cell  is  formed  by  the  nucleus.  These  cells, 
the  principal  elements  of  this  layer,  are  arranged  in  irregular  groups  ; 
they  are  multipolar,   and  have,   according  to  recent  investigations, 


306  NORMAL   HISTOLOGY. 

branched  protoplasmic  as  well  as  nervous  or  axis-cylinder 
processes  ;  while  the  former  ramify  among  the  cells  of  the  granule 
layer,  the  delicate  nervous  processes  extend  into  the  outer, 
molecular  layer,  where  they  usually  end  by  dividing  into  longitu- 
dinal T-branches  which  stretch  horizontally  parallel  with  the  boun- 
daries of  the  zone.  The  processes  of  these  cells  are  so  delicate,  as 
well  as  so  masked  by  the  surrounding  elements,  that  their  existence 
has  been  established  only  after  the  introduction  of  the  recent  methods 
of  Golgi,  the  results  of  whose  investigations  have  been  confirmed  by 
Ramon  y  Cajal,  Kolliker,  and  others.  Other  nervous  elements  of 
the  granule  layer  are  the  sparingly-distributed  multipolar  cells, 
much  larger  than  the  ones  just  considered,  which  resemble  in  struct- 
ure and  size  the  cells  of  Purkinje,  and,  like  them,  possess  richly- 
branched  protoplasmic  processes  extending  within  the  molecular 

Fig.  338. 


0  G  W 

Diagram  representing  cellular  constituents  of  cerebellar  cortex  ;  Golgi's  silver  staining :  W,  white 
matter  ;  O,  G,  outer  and  granule  layers  of  gray  matter;  a,  large  cell  of  granule  layer  confined  to  gray 
substance  ;  b,  6',  small  nerve-cells  of  granule  layer  (exaggerated  for  convenience),  also  limited  to  gray 
matter ;  c,  cell  of  Purkinje.  sending  axis-cylinder  into  granule  layer  and  richly-branched  processes 
towards  periphery  ;  e.  similar  cell  seen  in  profile  ;  /,  small  nerve-cell  of  outer  layer,  limited  to  gray 
matter;  g-,  nerve-cell  of  outer  layer,  whose  axis-cylinder  process  forms  basket-works  (d,  d')  around 
body  of  cells  of  Purkinje;  at  inner  border  of  outer  zone  numerous  horizontally  ramifying  branches 
of  nerve-fibres  are  seen. 


layer  ;  they  differ  in  the  distribution  and  form  of  the  axis-cylinder 
processes.  The  latter  are  directed  towards  the  medulla,  but,  instead 
of  passing  into  the  granule  layer  to  become  continuous  with  nerve- 
fibres,  the  processes  in  question  divide  and  subdivide  into  an  arbor- 
ization of  great  richness.  The  ramifications  of  the  two  varieties  of 
nerve-cells  of  the  granule  layer,  therefore,  are  distributed  in  a  manner 
directly  opposed,  the  nervous  processes  of  the  small  cells  terminating 


THE 


CENTRAL   NERVOUS   SYSTEM. 


307 


Fig.  339 


mv 


protoplasmic  processes  towards  periphery,  n, 
pia  (/). 


^0g  NORMAL   HISTOLOGY. 

within  the  outer  layer,  while  those  of  the  larger  cells  divide  within 
the  granule  layer  ;  in  both  cases,  it  will  be  remarked,  the  axis-cylinder 
processes  terminate  entirely  within  the  gray  matter,  thus  identi- 
fying their  possessors  as  nerve-cells  of  the  second  type.  In 
addition  to  the  nervous  elements,  a  few  flattened  cells,  with  feebly- 
developed  processes,  are  scattered  throughout  the  granule  zone  ; 
these  are  to  be  regarded  as  belonging  to  the  supporting  frame- 
work. The  interstices  between  the  numerous  nerve-cells  are  partly 
occupied  by  the  plexus  of  medullated  nerve-fibres  which  are 
derived  from  the  bundles  of  parallel  fibres  continued  from  the  medul- 
lary tracts  ;  some  of  these  fibres  pass  beyond  the  nuclear  layer  to 
end  within  the  molecular  zone. 

The  cells  of  Purkinje  form  the  thinnest  but,  at  the  same  time, 
the  most  characteristic  layer  of  the  cerebellar  cortex.  These  ele- 
ments, among  the  largest  ganglion-cells  in  the  body,  are  disposed  as 
a  single  row  at  the  junction  of  the  nuclear  and  the  molecular  layer, 
and  present  pyriform  or  flask-shaped  bodies,  60-70  p.  in  their  longest 
diameter,  placed  vertically  to  the  plane  of  the  zone,  with  the  larger, 
rounded  end  resting  on  the  outer  margin  of  the  nuclear  layer,  while 
the  smaller  end  is  directed  towards  the  periphery.  Each  cell  pos- 
sesses a  large  nucleus  (15  fi)  as  well  as  a  nucleolus,  and  differs 
from  other  ganglionic  elements  in  containing  little  or  no  pigment. 
The  central  pole  is  prolonged  as  the  axis-cylinder  process,  which, 
after  giving  off  collateral  fibres,  passes  on  to  become  the  axis-cylinder 
of  a  medullated  nerve.  The  most  distinctive  feature  of  these  cells, 
however,  is  the  distribution  of  their  protoplasmic  processes. 
A  thick  tapering  process,  usually  single,  but  occasionally  double, 
extends  from  the  small  end  of  the  flask-shaped  body  towards  the 
periphery  ;  this  stem  very  soon  divides  into  two,  the  branches  run- 
ning horizontally,  sometimes  almost  at  right  angles  to  the  parent 
stalk  before  turning  towards  the  surface  ;  the  peculiarity  of  the  rich 
ramification  which  follows  is  the  dominating  vertical  direction  of  the 
larger  branches.  While  the  pictures  presented  by  the  cells  of  Pur- 
kinje in  successfully-stained  sections  have  always  been  among  the 
most  striking,  it  was  not  until  the  introduction  of  Golgi's  silver 
method  that  a  full  appreciation  of  the  remarkable  richness  of  these 
ramifications  became  possible.  In  such  preparations  the  molecular 
layer  is  occupied  to  its  extreme  periphery  by  the  intertwining  but 
ununited  fibrils  of  the  branching  processes.  The  extent  and 
breadth  of  these  apparent  net-works,  however,  vary  with  the  point 
of  view,  for  the  cells  send  out  their  branches  especially  in  a  direction 
at  right  angles  to  the  long  axis  of  the  convolution  or  the  medullary 
tract,  while  in  a  plane  parallel  to  this  axis  the  branches  are  limited 
to  a  narrow  zone,  scarcely  wider  than  the  body  of  the  cell :  it  follows 


THE   CENTRAL    NERVOUS    SYSTEM. 


309 


that  in  order  to  display  Purkinje's  cells  to  the  best  advantage  the 
tissue  should  be  sectioned  across,  and  not  parallel  with,  the  axis 
of  the  convolutions.  These  cells,  further,  are  not  placed  at  uniform 
distances  throughout  the  row  which  they  form,  but  are  more  numerous 

Fig.  340. 


Section  of  outer  portion  of  cerebellar  cortex  of  young  dog,  stained  after  Golgi's  silver  method : 
P,  cell  of  Purkinje,  exhibiting  profuse  arborization  of  protoplasmic  processes;  /,  its  axis-cylinder 
process ;  B,  B,  cells  of  outer  layer  whose  axis-cylinder  processes  form  basket- works  around 
bodies  of  Purkinje's  cells ;    C,  small  ganglion-cells  limited  to  outer  layer.     (After  Retzius.) 


and  more  closely  arranged  at  the  summit  of  the  convolutions, 
at  the  bottom  of  the  fissures  being  more  widely  separated ;  these 
variations  correspond  with  the  areas  of  greatest  and  least  development 
of  the  nuclear  layer. 

The  molecular  or  outer  layer  consists  of  a  ground-substance 
of  finely-reticulated  supporting  neuroglia,  in  which  extend  the  elab- 
orate arborizations  of  Purkinje's  cells,  together  with  certain 
nervous  elements  belonging  to  this  zone.  These  latter  are  of  two 
kinds  :  small  multipolar  cells  whose  branched  protoplasmic  pro- 
cesses extend  towards  the  periphery,  while  the  nervous  process  is 
directed  centrally,  but  probably  is  confined  to  the  molecular  layer, 
and  larger  elements  distinguished  by  the  remarkable  termination 
of  their  axis-cylinder  or  nervous  processes.     While  the  protoplasmic 


,1Q  NORMAL   HISTOLOGY. 

processes  ramify  within  the  outer  part  of  the  molecular  layer,  the 
axis-cylinder  process,  after  a  short  course,  passes  horizontally 
near  the  margin  of  the  large-celled  layer,  and,  at  various  intervals, 
sends  off  lateral  branches  which  subdivide  to  form  net-works  of 
fibrils,  the  fibre-baskets,  or  basket-works,  around  the  bodies 
of  Purkinje's  cells.  In  addition  to  the  nervous  elements,  cells 
belonging  to  the  neuroglia  are  scattered  throughout  the  zone.  As 
already  stated,  certain  of  the  nerve-fibres  entering  the  granule  zone 
continue  into  the  outer  layer  ;  these  fibres,  after  penetrating  for  a 
short  distance,  divide  into  terminal  branches,  many  of  which 
extend  horizontally,  parallel  to  the  boundaries  of  the  zone,  to  end 
free,  in  close  relation  but  without  direct  continuity  with  the 
nerve-cells. 

In  addition  to  the  peripheral  cortical  layer,  the  cerebellum  possesses 
other  masses  of  gray  matter,  the  central  nuclei,  embedded  within 
the  medullary  substance  of  the  vermiform  process  and  of  the  adjacent 
parts  of  the  hemispheres.  The  central  nuclei  are  two  :  the  nucleus 
dentatus,  situated  within  the  hemisphere  of  each  side,  and  the 
nuclei  of  the  roof,  within  the  worm  ;  the  nucleus  emboliformis 
and  nucleus  globosus,  sometimes  described  as  separate  centres,  are 
really  parts  of  the  complicated  dentate  nucleus. 

The  dentate  nucleus  consists  of  a  greatly  plicated  pouch-like 
sheet  of  gray  substance,  .3—5  mm.  in  thickness,  situated  within  the 
fibre-tract  of  the  superior  peduncle  of  the  cerebellum.  The  nerve- 
cells  contained  within  the  band  are  of  moderate  size  (25-35  P-)  and 
pigmented  to  a  variable  degree  ;  the  loosely-packed  cells  possess 
branched  processes  extending  outward,  and  an  axis-cylinder 
process  directed  towards  the  medulla.  Numerous  nerve-fibres 
pass  between  the  cells  and  connect  the  white  core  within  the  nucleus 
with  the  surrounding  medullary  substance. 

The  nuclei  of  the  roof  consist  of  irregularly  ovoid  areas  of  gray 
substance  (6-8  mm.  in  length)  situated  within  the  vermiform  process, 
almost  in  contact  along  the  mesial  line.  The  masses  contain  large 
pigmented  multipolar  ganglion-cells  (45-80  mm.)  and  numerous 
nerve-fibres,  some  of  which  are  exceptionally  large. 

The  medullary  or  white  substance  of  the  cerebellum  em- 
braces the  numerous  bundles  of  nerve-fibres  which  maintain  the 
intricate  and  far-reaching  communications  of  this  division  of  the 
brain.  The  cerebellar  fibres  are  arranged  in  three  principal 
tracts,  the  cerebellar  peduncles ;  the  lower  of  these  corre- 
sponds to  the  corpus  restiforme,  the  middle  to  the  pedunculi 
pontis,  and  the  upper  to  the  processus  cerebelli  ad  corpora 
quadrigemina. 

The  fibres  of  the  white  matter  are  disposed  in  thin  flat  bundles, 


THE   CENTRAL   NERVOUS   SYSTEM.  ,ir 

which  diverge  from  the  chief  stem  as  the  primary  and  secondary- 
medullary  branches  ;  these  form  the  "  arbor  vitae." 

The  blood-vessels  supplying  the  cerebellum,  principally  branches 
of  the  vertebral  and  basilar  arteries,  after  repeated  division  within 
the  pia,  send  small  branches  vertically  into  the  molecular  layer 
as  far  as  its  inner  boundary ;  a  rich  vascular  net-work  surrounds 
the  cells  of  Purkinje  :  while  capillaries  are  wanting  within  the  pe- 
ripheral zone  of  the  molecular  layer,  they  are  well  represented 
in  the  granule  layer  and  among  the  nerve-bundles  of  the  me- 
dulla, where  the  blood-vessels  run  between  the  fibres  and  form  elon- 
gated meshes  which  correspond  to  the  disposition  of  their  tracts. 

THE    CEREBRUM. 

The  cerebral  hemispheres  consist  of  a  thin  outer  sheet  of  gray 
matter,  the  cortex,  which  everywhere  covers  in  the  white  matter  of 
the  medulla,  accurately  following  the  intricacies  of  the  convoluted 
surface  of  the  brain ;  in  addition  to  the  cortex,  large  special  masses 
of  gray  matter  lie  within  the  medullary  substance  and  take  part  in 
the  constitution  of  the  nucleus  caudatus,  the  nucleus  lenticu- 
laris,  the  thalamus,  the  corpus  subthalamicum,  and  the  minor 
collections  of  lesser  importance. 

The  cerebral  cortex  forms  a  dark,  peripheral  zone,  2-4  mm.  in 
thickness,  which  is  best  developed  in  the  ascending  frontal  and  the 
paracentral  convolution,  being  thicker  at  the  summit  of  the  gyri  than 
in  the  fissures  ;  the  gray  stratum  appears  least  conspicuous  in  the 
posterior  part  of  the  occipital  lobe. 

The  arrangement  of  the  elements  of  the  cortex  in  layers  is  indi- 
cated by  the  stratification  which  the  vertically-cut  surface  of  the 
cortex  presents  even  to  the  unaided  eye  ;  in  favorable  situations 
three  bands  are  distinguishable,  an  outer  white,  a  middle  gray, 
and  an  inner  yellowish  red ;  in  certain  regions,  as  the  superior 
frontal,  the  precentral,  and  the  occipital  convolutions,  the  layers  are 
increased  to  six  by  the  addition  of  the  stripes  of  Baillarger.  These 
markings,  however,  do  not  accurately  represent  the  structure  of 
the  cortex,  which  can  be  studied  adequately  only  in  successfully- 
stained  sections  cut  vertically  to  the  free  surface  of  the  convolution 
and  parallel  with  the  general  course  of  the  nerve-fibres.  In  such 
preparations  five  zones  are  recognizable,  which,  however,  are  not 
sharply  defined  from  one  another,  but  are  often  blended. 

1.  The  first  or  outer  layer,  next  the  pial  surface,  about  .25  mm. 
in  thickness,  is  composed  essentially  of  neuroglia,  together  with 
numberless  delicate  terminal  ramifications  of  the  protoplasmic 
processes  of  nerve-cells  situated  within  the  deeper  layers,  and  a 
few  tangential  nerve-fibres  ;  the  protoplasmic  threads  contributed 


312 


NORMAL    HISTOLOGY. 


Fig.  i 


by  the  cells  are  so  plentiful  and 
closely  interwoven  that  they  con- 
stitute no  inconsiderable  part  of 
the  fine  ground  reticulum  of  this 
layer.  Immediately  beneath  the 
surface  of  the  nervous  matter,  the 
sub-pial  zone  forms  a  narrow 
stratum  (10-25  //)  composed  al- 
most entirely  of  neuroglia,  in 
which  lie  numbers  of  spider  or 
Deiters's  cells.  The  nerve-fibres 
of  this  layer  extend  parallel  with 
the  free  surface. 

2.  The  second  layer  (.25 
mm.)  is  characterized  by  the 
profusion  of  its  closely- packed 
small  triangular  or  pyramidal 
nerve-cells,  the  branched  pro- 
toplasmic processes  of  which 
extend  in  various  directions  to- 
wards the  periphery,  while  their 
axis-cylinder  processes  termi- 
nate within  the  gray  matter,  often 
ending  in  T-branches  which  are 
directed  almost  at  right  angles  to 
the  main  process. 

3.  The  third  layer,  the  for- 
mation of  the  cornu  Ammonis, 
is  the  thickest  stratum  of  the 
cerebral  cortex,  reaching  in  places 
a  breadth  of  1  mm.,  and  contains 
the  most  characteristic  nervous 
elements  of  the  cerebrum,  the 
large  pyramidal  ganglion- 
cells.  This  layer  is  not  sharply 
defined  from  the  preceding,  since 
the  small  cells  of  the  latter  are  grad- 
ually replaced  by  the  larger  pyram- 


Section  of  human  cerebral  cortex  stained  with 
sodium  carminate  :  A,  outer  layer,  poor  in  nerve- 
cells,  rich  in  neuroglia  ;  B,  layer  of  numerous 
small  nerve-cells  ;  C,  layer  of  large  pyramidal  gan- 
glion-cells ;  D,  layer  of  irregular  numerous  but 
small  nerve-elements  ;  /,  pial  tissue  ;  v,  v' ,  blood- 
vessels. 


THE   CENTRAL   NERVOUS   SYSTEM. 


313 


Fig.  342. 


idal  elements,  which  become  more  widely  separated  and  of  greater 
size  on  approaching  the  deeper  parts  of  the  zone ;  in  this  situation 
their  basal  diameter  may  reach  40-50  fi. 
The  pyramidal  cells,  in  addition  to 
the  general  outlines  of  their  bodies, 
are  distinguished  by  the  arrange- 
ment of  their  processes  ;  the  pro- 
toplasmic ramifications  are  disposed 
as  the  principal  apical  processes, 
which  extend  towards  the  periphery 
as  far  as  the  sub-pial  zone  (Retzius) 
and  by  repeated  division  form  a  rich 
arborization  within  the  outer  layers 
of  the  cortex,  and  as  lateral  basal 
processes,  which  pass  obliquely  from 
the  base  and  break  up  into  rich  net- 
works of  delicate  terminal  protoplas- 
mic threads  ;  in  addition,  numerous 
smaller  lateral  processes  are  given 
off  from  the  sides  of  the  cell.  Not- 
withstanding the  profusion  of  the 
fibrils  resulting  from  the  subdivision 
of  the  protoplasmic  processes  of  these 
cells,  it  is  highly  probable  that  the 
fibrils  terminate  without  uniting 
with  one  another.  From  the  blunt, 
central  end  of  the  cell  the  axis -cyl- 
inder process  extends  into  the  white 
matter,  where  it  becomes  continuous 
with  a  nerve-fibre.  These  axis-cyl- 
inder prolongations  give  off  recurrent 
collateral  processes,  which  bend 
towards  the  periphery.  The  pyrami- 
dal body  of  the  cell  contains  a  large 
round  or  oval  nucleus,  with  a  dis- 
tinct nucleolus,  embedded  within  a 
finely  granular  protoplasm,  masses  of 
brownish  pigment  almost  always  occupying  the  base  of  the  cell. 
The  larger  pyramidal  cells  are  surrounded  by  pericellular  lymph- 
spaces,  which  probably  communicate  with  the  extensions  of  the 
subarachnoidean  space  continued  with  the  prolongations  of  the 
pia  accompanying  the  blood-vessels  within  the  cerebral  tissue. 

4.   The  fourth  layer  embraces  a  closely-packed  zone  (.3-4  mm.) 
composed  of  small,  irregular,  oval  or  angular  nerve-cells,  7- 


Section  of  cerebral  cortex  (motor  area) 
of  child  stained  by  Golgi's  silver  method  : 
A,  layer  of  neuroglia-cells ;  B,  layer  of 
small  pyramidal  ganglion-cells  ;  C,  layer 
of  large  pyramidal  cells ;  D,  layer  of  ir- 
regular smaller  cells. 


3'4 


NORMAL   HISTOLOGY. 


14  ,'j.  in  diameter  ;  among  the  smaller  elements  a  few  larger  pyramidal 
cells  are  often  encountered,  as  well  as  radiating  bundles  of  med- 
ullated  nerve-fibres.  The  cells  of  this  layer  resemble  those  of 
the  second,  since  their  axis-cylinder  processes  are  confined  to  the 

gray  matter,  the  elements  be- 


Fig 


cells    of  the 


ing,    therefore, 
second  type. 

5.  The  fifth  layer  indicates 
the  proximity  of  the  white 
matter  by  the  large  areas  occu- 
pied by  bundles  of  radiating 
nerve-fibres  directly  contin- 
uous with  the  medullary  tracts  ; 
within  the  interspaces  between 
the  nerve-bundles  lie  the  small 
and  medium  -  sized  cells, 
spindle  to  pyramidal  in  form, 
which  characterize  this  layer. 
While  these  cells  are  arranged 
generally  parallel  with  the  fibre- 
bundles,  sometimes,  especially 
at  the  bottom  of  the  fissures, 
they  are  placed  at  right  angles 
thereto,  in  the  latter  case  as- 
suming a  pronounced  spindle 
type. 

The  nerve-fibres  enter- 
ing the  gray  cortex  are  ar- 
ranged in  bundles,  from  which 
arise  net  -  works  variously 
situated  and  arranged.  The 
radial  bundles  proceed  as 
such  through  about  half  the 
entire  thickness  of  the  cortex  ; 
beyond  this  level  they  rapidly 
separate  into  the  component 
fibres  which  take  their  way 
between  the  ganglion  -  cells. 
The  fibres  given  off  during  the 
course  of  the  bundles  form  net- 
works at  all  depths  occupying  the  interfascicular  portions  of  the 
layers  traversed  ;  within  the  deepest  part  of  the  fourth  layer,  how- 
ever, the  nervous  fibrillae  are  especially  numerous,  and  constitute 
a  conspicuous    reticulum   in  preparations  stained  by  Weigert's 


Section  of  human  cerebral  cortex  stained  by 
Weigert's  method,  exhibiting  groups  of  nerve-fibres ; 
part  of  white  matter  and  inner  layers  of  gray  sub- 
stance shown  :  F,  white  matter  from  which  radiating 
bundles  of  nerve-fibres  (n)  extend  into  gray  matter  ; 
C,  D,  and  E,  third,  fourth,  and  fifth  zones  of  gray 
matter  :  cells  are  faintly  stained. 


THE    CENTRAL    NERVOUS    SYSTEM. 


315 


method.  In  the  deeper  parts  of  the  broad  third  layer  a  similar 
well-marked  net-work  occurs,  the  interlacing  fibres  of  which  sur- 
round the  nerve-cells  of  the  layer.  Beyond  this  plane  nervous 
reticulations  occupy  the  third  and  second  layers  and  extend  into 
the  outer  zone. 

A  limited  number  of  the  nerve-fibres  terminate  within  the  outer 
layer  as  axis-cylinders  which  run  parallel  with  the  free  surface, 
as  do  also  the  terminal  ramifications  of  the  axis-cylinder  processes 
of  some  of  the  ganglion-cells.  The  recent  investigations  of  Golgi 
and  others  have  shown  that  many  fibres  end  without  demonstrable 
direct  anatomical  continuity  with  the  nerve-cells,  although  a 
close  relation  between  the  cells  and  the  fibres  undoubtedly  exists. 

While  the  arrangement  just  described  may  be  regarded  as  typical 
for  the  greater  part  of  the  cortex,  a  few  localities  are  distinguished 
by  modifications  which  materially  affect  the  histological  details. 
These  changes  depend  upon  either  an  arrested  development  of  the 
cortex,  as  in  the  septum  lucidum,  or  an  increased  complexity  of 
the  cortical  arrangement,  as  in  the  hippocampal  convolution.  Less 
conspicuous  variations,  affecting  one  or  more  layers,  are  frequently 
encountered ;  thus,  the  paracentral  convolution  contains  the 
largest  pyramidal  cells,  the  "giant  pyramids"  (Betz),  the  entire 
third  layer  participating  in  the  increase  of  size.  The  occipital 
cortex  is  especially  differentiated  by  subdivisions  of  the  third  and 
fourth  layers  into  eight  layers  (Meynert),  while  the  gyrus  cinguli 
has  the  third  layer  separated  into  an  outer  group  of  small  and  an 
inner  zone  of  larger  cells,  the  intervening  space  appearing  radially 
striated  on  account  of  the  apical  processes  which  cross  it ;  within  the 
parietal  lobes  an  additional  stratum  of  small  pyramidal  cells  exists 
between  the  third  and  fourth  cortical  layers. 

The  involuted  cortex  of  the  hippocampal  region,  including  the 
cornu  Ammonis,  or  hippocampus  major,  and  the  fascia  dentata, 
presents  considerable  complexity.  On  observing  a  section  of  this 
region  with  low  amplification,  it  will  be  seen  that  the  cornu  Ammo- 
nis consists  of  a  central  gray  zone  bounded  both  internally  and 
externally  by  a  stratum  of  white  substance  ;  the  gray  zone 
corresponds  to  the  cortex  of  other  parts,  and  is  continuous  with 
the  thickened  gray  mass  constituting  the  fascia  dentata  above,  and 
with  the  cortex  of  the  hippocampal  convolution  below.  The 
medullary  substance  of  the  latter  becomes  greatly  reduced  in  its 
passage  over  the  cornu  Ammonis,  the  attenuated  stratum  of  fibres 
being  known  as  the  alveus  which  is  prolonged  into  the  thicker 
fimbria.  The  white  layer  enclosing  the  gray  zone  on  the  mesial 
surface  is  a  conspicuous  thickening  of  the  peripheral  zone  of  the 
hippocampal  convolution. 


^j5  normal  histology. 

The  several  structures  composing  the  cornu  Ammonis,  examined 
from  the  ventricle  towards  the  outer  surface,  are — 

i.  The  alveus,  an  attenuated  layer  of  medullated  nerve-fibres, 
homologous  with  the  medullary  substance  of  the  typical  convolu- 


Fig.  344. 
Fi 


m^mm 


^■^mw,A 


o 


Section  across  cornu  Ammonis,  fascia  dentata,  and  fimbria  :  Gh,  hippocampal  convolution  ; 
Fd,  fascia  dentata,  separated  from  preceding  by  hippocampal  fissure  ;  Fi,  fimbria,  composed 
of  transversely- cut  longitudinal  nerve-fibres;  1,  2,  medulla  of  hippocampal  convolution  con- 
tinued over  cornu  Ammonis  ( C),  as  alveus,  into  fimbria  ;  3,  layer  of  large  pyramidal  cells  ;  4, 
stratum  radiatum  ;  5,  stratum  lacunosum ;  6,  stratum  moleculare ;  7,  lamina  medullaris 
involuta  ;  m,  termination  of  this  lamina  in  longitudinal  fibres  ;  «,  nucleus  fasciae  dentatae ;  g, 
stratum  granulosum ;  r,  reticulated  neuroglia-layer  covered  by  thin  sheet  of  nerve-fibres. 
(After  Henle.) 


tion.  The  fibres,  while  pursuing  a  course  generally  parallel  to  the 
ventricular  surface,  run  somewhat  obliquely  ;  on  approaching  the 
fimbria  the  layer  increases  in  thickness  and  the  nerve-fibres  assume 
a  disposition  less  oblique,  until,  within  the  fimbria,  their  direction 
almost  coincides  with  the  long  axis  of  the  cornu  Ammonis. 


THE    CENTRAL    NERVOUS    SYSTEM. 


317 


2.  The  stratum  oriens,  representing  the  fifth  layer  of  the  cortex, 
and  containing  among  the  bundles  of  nerve-fibres  numbers  of  spindle- 
form  cells,  whose  processes  extend  parallel  with  the  free  surface. 

3.  The  stratum  cellularum  pyramidalium,  which  corresponds 
to  the  deeper  portions  of  the  third  cerebral  layer,  and  is  conspicuous 
on  account  of  the  large  pyramidal  ganglion-cells.  The  latter, 
moderate  in  size  (30-40  /->.),  are  arranged  in  several  closely-packed 
rows,  and  send  their  axis-cylinder  processes  into  the  adjoining 
medullary  substance  of  the  alveus,  while  their  long  apical  proto- 
plasmic processes  pass  towards  the  periphery  and  give  to  the  outer 
part  of  the  third  layer  a  vertical  striation,  which  has  received 
recognition  as 

4.  The  stratum  radiatum.  This  layer  consists  almost  entirely 
of  the  long,  tapering  processes  of  the  pyramidal  elements, 


Fig.  345. 


Diagram  of  the  constituents  of  the  cornu  Ammonis,  Golgi  staining  :  H ,  hippocampal  convolution  ; 
C,  cornu  Ammonis ;  F,  fascia  dentata ;  i,  fusiform,  2,  3,  small  and  irregular,  4,  5,  pyramidal,  and  6, 
small,  cells  of  respective  layers;  7,  8,  nerve-cells  of  fascia  dentata;  al,  collaterals  of  pyramidal  cells; 
course  of  axis-cylinder  processes  shown  by  fine  lines.     (After  Karl  Schaffer.) 


which  often  show  a  disposition  to  divide  into  numerous  branches 
before  reaching  the  border  of  the  zone. 

5.  The  stratum  lacunosum,  composed  principally  of  axis-cyl- 
inders, which  extend  parallel  to  the  fibre-layer  of  the  alveus,  to- 
gether with  the  collateral  processes  from  the  neighboring  nerve-cells. 

6.  The  stratum  moleculare,  which  contains  sparingly  distributed 
fusiform  or  pyramidal  ganglionic  elements,  whose  protoplas- 


-xg  NORMAL   HISTOLOGY. 

mic  processes  extend  vertically  into  the  outer  part  of  the  zone  of 
pyramidal  cells,  as  well  as  laterally  within  the  molecular  layer ;  their 
axis-cylinder  processes,  on  the  contrary,  are  directed  towards  the 
peripheral  nerve-fibres,  among  which  they  end. 

7.  The  lamina  medullaris  involuta,  constituting  the  outermost 
layer  of  the  convolution,  lying  next  the  fascia  dentata,  from  which  it 
is  separated  by  the  intervening  hippocampal  fissure  and  its  pial  fold. 
This  layer  corresponds  to  the  greatly  thickened  outer  zone  of  the 
usual  cortex,  and  is  largely  made  up  of  tangential  nerve-fibres 
which  proceed  from  the  gyrus  hippocampi,  together  with  numerous 
terminal  fibrillse  derived  from  the  processes  of  ganglion-cells 
situated  in  neighboring  strata. 

The  fascia  dentata  must  be  regarded  as  the  projecting  thickened 
and  specialized  free  edge  of  the  cortical  gray  matter,  lodged 
within  the  hippocampal  fissure,  which  it  almost  fills.  The  divisions 
recognizable  in  this  structure,  from  within  outward,  are — 

1.  The  nucleus  fasciae  dentatae,  which  comprises  an  oval  area 
containing  nerve-fibres  continued  from  the  alveus  and  numerous 
ganglion-cells.  The  latter  include  three  varieties  of  irregularly- 
disposed  elements,  the  pyramidal  cells  proper,  the  representatives 
of  the  similar  conspicuous  constituents  of  the  cornu  Ammonis,  the 
polymorphous  cells,  possessing  very  numerous  processes,  and  the 
fusiform  cells. 

2.  The  stratum  granulosum,  distinguished  as  a  conspicuous 
band  of  brilliantly  staining  small  pyriform  nervous  elements, 
whose  protoplasmic  processes  extend  towards  the  periphery,  while 
the  axis-cylinder  fibrils  in  general  pass  centrally. 

3.  The  stratum  moleculare,  consisting  of  a  broad  reticulated 
zone  of  neuroglia,  which  contains  numerous  capillary  blood-vessels, 
a  few  scattered  cells,  and  the  extensions  of  the  processes  of  the  nerve- 
cells  ;  it  almost  completely  encloses  the  stratum  granulosum,  and  is 
itself  covered  by 

4.  The  stratum  marginale,  an  extremely  thin  sheet  of  medul- 
lated  nerve-fibres  representing  the  outer  medullary  layer  of  the 
cornu  Ammonis,  of  which  it  is  the  direct,  although  attenuated,  con- 
tinuation. 

The  fimbria  receives  the  fibres  constituting  the  alveus,  and  is  com- 
posed entirely  of  bundles  of  medullated  nerve-fibres,  together 
with  the  intervening  connective-tissue  septa  ;  the  thick  fibre-bundles 
extend  longitudinally,  and  are  continued  into  the  tracts  of  the  posterior 
pillar  of  the  fornix. 

The  septum  lucidum  represents  a  rudimentary  cortex  due  to 
the  arrest  of  development  following  the  isolation  of  this  part  of  the 
wall  of  the  cerebral  vesicle  by  the  growth  of  the  corpus  callosum. 


THE    CENTRAL    NERVOUS    SYSTEM.  3IQ 

Since  the  so-called  fifth  ventricle  is  really  a  cut-off  portion  of  the 
great  longitudinal  fissure,  those  surfaces  directed  towards  the  cleft 
correspond  to  the  free  surface  of  the  hemispheres  ;  the  rudimentary 
layer  of  gray  matter,  therefore,  forms  the  immediate  lateral  boun- 
daries of  this  space,  and  is  homologous  with  the  cortex  of  other 
regions,  while  the  thin  white  stratum  next  the  lateral  ventricles 
represents  the  medulla. 

The  mesially-placed  gray  cortex  of  the  septum  lucidum  con- 
tains a  thin  superficial  stratum  of  medullated  nerve-fibres  next 
the  interseptal  cleft  ;  following  this  lies  a  layer  of  gray  matter  con- 
taining many  small  pyramidal  cells  (16— iS  ft),  the  apical  processes 
of  which  are  directed  towards  the  surface  homologous  with  the  pe- 
riphery of  the  hemispheres  ;  the  deeper  zone  of  the  gray  matter 
exhibits  spindle-cells.  The  white  substance  of  the  hemisphere  is 
represented  by  the  thin  stratum  of  medullated  fibres  interposed 
between  the  gray  layer  and  the  ependyma  of  the  lateral  ventricle. 

The  blood-vessels  supplying  the  cerebral  cortex,  after  a  short 
course  within  the  pia  almost  parallel  to  the  free  surface,  enter  the 
nervous  tissue  vertically  ;  the  larger  arteries  pierce  the  gray  matter 
and  enter  the  medulla,  while  those  of  smaller  size  break  up 
within  the  gray  cortex  into  capillary  net-works.  The  law,  appli- 
cable to  all  parts  of  the  nervous  system,  that  regions  rich  171  large 
nerve-cells  are  plentifully  supplied  with  blood-vessels,  is  illustrated 
by  the  distribution  of  the  capillaries  within  the  cortex,  where  a  rich 
capillary  net-work  exists  within  the  layer  of  large  pyramidal  cells, 
while  the  outer  cortical  zones,  on  the  contrary,  possess  only  a  meagre 
capillary  supply  :  the  net-works  within  the  deepest  layers  of  the 
cortex  are  intermediate  in  the  closeness  of  their  meshes.  The  blood- 
vessels are  surrounded  by  perivascular  lymph-spaces,  the  pial 
tissue  accompanying  the  vessels  as  a  delicate  sheath  attached  to  the 
adventitia  and  enclosing  a  prolongation  of  the  subarachnoidean 
space. 

The  corpus  striatum  consists  of  the  special  masses  of  gray  matter, 
the  nucleus  caudatus  and  the  nucleus  lenticularis,  and  their 
associated  tracts  of  nerve-fibres. 

The  ventricular  surface  of  the  nucleus  caudatus  is  covered 
by  a  well-developed  layer  of  ependyma,  beneath  which  lies  the 
zone  of  gray  substance,  containing  nerve-cells  of  two  kinds  :  large 
multipolar  cells  (25-30  fi),  and  much  more  numerous  smaller 
ganglionic  elements,  whose  size  is  about  half  that  of  the  former. 
The  outer  surface  of  the  caudate  nucleus,  directed  towards  the 
internal  capsule,  is  broken  up  by  numerous  bundles  of  fibres,  which 
penetrate  deeply  into  the  gray  mass  and  produce  the  characteristic 
white  striae  exhibited  on  section. 


320 


NORMAL   HISTOLOGY. 


The  outer  division,  or  the  putamen,  of  the  nucleus  lenticu- 
laris  closely  resembles,  both  in  color  and  in  structure,  the  caudate 
nucleus,  with  which,  indeed,  it  anteriorly  becomes  continuous.  The 
paler  color  of  the  inner  segments,  the  globus  pallidus,  depends 


l.m1 


co. a. 


Section  across  anterior  end  of  thalamus,  striatum,  and  insula:  th,  anterior  end  of  thalamus  ;  sit., 
stria  terminalis  ;  n.c,  nucleus  caudatus  ;  «./.,  outer  segment  of  nucleus  lenticularis  ;  l.m,,  l.m.',  ex- 
ternal and  internal  medullary  lamina  receiving  fibres  (.r)  from  caudate  nucleus  ;  c.i.,  internal  capsule  ; 
c.e.,  external  capsule;  cl,  claustrum  ;  co,  cortex  of  island  of  Reil ;  co.a.,  anterior  commissure;  g, 
central  gray  matter  of  third  ventricle,  a,  its  commissure;  c.f.,  section  of  anterior  pillar  of  fornix; 
b,  c,  d,  e,  elements  of  subthalamic  region  ;  e1,  stratum  zonale  of  thalamus  ;  o,  portion  of  optic  tract. 
(After  Schwalbe-Meynert.) 

not  only  upon  the  presence  of  greater  numbers  of  medullated  fibres, 
but  also  upon  the  lighter  tint  of  the  yellowish  pigment  contained 
within  the  multipolar  nerve-cells.  The  nerve-cells  contained  within 
the  claustrum  are  principally  fusiform  elements  whose  long 
axes  correspond  in  direction  with  the  neighboring  free  surface. 

The  optic  thalamus  is  composed  chiefly  of  gray  matter,  through 
which  extend  various  tracts  of  nerve-fibres.  The  surfaces  directed 
towards  the  ventricles  are  sharply  defined,  the  upper  or  dorsal 
aspect  being  covered  by  a  layer  of  medullated  nerve-fibres,  the 
stratum  zonale,  about  .8  mm.  in  thickness,  which  fade  away  on  the 
mesial  surface  ;  the  outer  and  ventral  borders  of  the  thalamus,  on 
the  contrary,  are  invaded  by  fibres  from  the  respectively  adjacent 
internal  capsule  and  the  subthalamic  region. 

The  gray  matter  of  the  thalamus  is  divided  by  tracts  of  fibres 
into  a  shorter  median  segment,  the  inner  nucleus,  and  a  longer 


THE    CENTRAL    NERVOUS    SYSTEM. 


321 


external  division,  the  outer  nucleus,  the  fore-segment  of  the  thala- 
mus containing  the  anterior  or  upper  nucleus.  The  gray  sub- 
stance composing  these  segments  is  traversed  in  many  places  by 
bundles  of  medullated  nerve-fibres  ;  in  the  outer  nucleus  the  nar- 
row fibre-bundles  and  the  zones  of  gray  substance  alternate,  fusi- 
form ganglion-cells  (20-30  //),  arranged  parallel  to  the  course  of 
the  fibres,  occupying  the  gray  bands.  In  addition  to  the  compara- 
tively large  cells  found  within  the  anterior  nucleus,  the  bundle  of 
Vicq  d' Azyr,  reflected  from  the  mammillary  body  below,  enters  the 
anterior  ventral  border  of  the  segment  and  contributes  numerous 
fibres  to  its  mass.  The  multipolar  cells  of  the  anterior  nucleus, 
as  well  as  those  of  the  posteriorly  situated  pulvinar,  are  of  large 
size. 

The  central  gray  matter  of  the  third  ventricle  is  the  direct 
continuation  of  that  lining  the  Sylvian  aqueduct  and  other  parts 
of  the  neural  tube.  The  middle  or  gray  commissure  of  the 
ventricle  contains  transverse  fibres,  in  addition  to  numerous 
pigmented  ganglion-cells ;  posteriorly  it  is  intimately  blended 
with  the  gray  substance  of  the  thalamus,  while  anteriorly  it  is  sepa- 
rated from  the  latter  by  a  medullary  layer,  the  inferior  stalk  of  the 
thalamus. 

The  corpus  subthalamicum,  situated  within  the  region  of  simi- 
lar name,  is  composed  of  a  very  close  net-work  of  fine  medul- 
lated fibres,  among  which  are  distributed  moderate-sized  multi- 
polar nerve-cells  ;  the  capillary  net-work  of  this  nucleus  is 
remarkable  for  the  closeness  of  its  meshes.  The  continuation  of  the 
area  of  pigmented  cells  forming  the  substantia  nigra  within  the 
cerebral  crus  separates  the  subthalamic  region  from  the  fibre-tracts  of 
the  crusta. 

The  corpora  quadrigemina,  the  homologues  of  the  optic  lobes 
and  corpora  bigemina  of  the  lower  animals,  comprise  a  posterior 
and  an  anterior  pair  of  eminences. 

The  posterior  quadrigeminal  bodies  consist  in  great  part  of 
gray  substance  which  forms  a  lenticular  nucleus  on  either  side,  and 
contains  numerous  small  multipolar  cells  (16-18  /*),  as  well  as  a  few 
elements  of  larger  size  ;  the  nuclei  of  the  two  sides  are  united  by  a 
gray  commissure.  A  thin  superficial  lamina  of  medullated  nerve- 
fibres,  the  stratum  zonale,  covers  in  the  gray  matter. 

The  anterior  quadrigeminal  bodies  differ  from  the  posterior 
in  the  complexity  of  their  structure  brought  about  by  the  presence 
of  the  root-tracts  of  the  optic  nerve.  In  transverse  section  these 
bodies  present  four  layers,  which,  from  the  upper  or  dorsal  surface 
towards  the  Sylvian  canal,  are — 

1.  The  stratum  zonale,  enveloping  the  superficial  portions  of 


322 


NORMAL    HISTOLOGY. 


the  bodies  ;  in  man  and  apes  this  layer  is  unusually  well  developed, 
reaching  a  thickness  of  30-40  ft ;  the  interlacing  fibres,  derived 
from  the  optic  tract,  form  a  lamina  rather  than  distinctly-grouped 
bundles. 

2.  The  stratum  cinereum,  a  cap-like  mass  of  gray  matter,  em- 
bracing the  subjacent  optic  fibres,  and  containing  numerous  nerve- 
cells  of  varying  size,  the  larger  ones  occupying  the  deeper  parts  of 
the  layer. 

3.  The  stratum  opticum,  consisting  of  the  continuation  of  the 
preceding  gray  matter,  through  which  extend  the  bundles  of  optic 
nerve-fibres  entering  by  the  superior  brachium  ;  posteriorly  the  fibres 
are  fine,  while  anteriorly  they  become  robust  to  take  part  in  the 


Fig.  347. 


c.frs 


Section  across  superior  corpora  quadrigemina  and  adjacent  part  of  thalamus  :  i, 
Sylvian  aqueduct ;  gr,  gray  matter  of  aqueduct ;  c.q.s.,  quadrigeminal  body,  con- 
sisting of,  /,  stratum  lemnisci,  o,  stratum  opticum,  c,  stratum  cinereum  ;  Th,  thala- 
mus (its  pulvmar)  ;  c.g.i.,  c.g.e.,  internal  and  external  geniculate  bodies  ;  br.s.,  br.i., 
superior  and  inferior  brachia  ;  /,  upper  fillet ;  /./.,  posterior  longitudinal  bundle  ;  r, 
raphe  ;  ///,  third  nerve  ;  n.III,  its  nucleus;  l.p.p.,  posterior  perforated  space;  s.n., 
substantia  nigra,  above  this  tegmentum  with  circular  nucleus  ;  cr,  crusta  ;  //,  optic 
tract;  M,  medulla  of  hemisphere;  n.c,  nucleus  caudatus  ;  si,  stria  terminalis. 
(After  Quain-Meynert .) 


constitution  of  the  optic  tract.  Among  the  profusely  distributed 
nerve-cells  are  elements  of  considerable  size  whose  axis-cylinder 
processes  extend  largely  within  the  underlying  zone. 

4.  The  stratum  lemnisci,  including  gray  matter  as  well  as  nerve- 
fibres,  many  of  the  latter  being  continuations  of  tracts  connecting 
lower  levels. 

The  geniculate  bodies,  lateral  and  mesial,  are  closely  associated 


THE    CENTRAL    NERVOUS    SYSTEM.  ,2, 

respectively  with  the  anterior  and  the  posterior  corpora  quadrigemina 
by  means  of  the  corresponding  brachia. 

The  lateral  or  external  geniculate  bodies  exhibit  a  character- 
istic structure,  consisting  of  alternate  layers  of  white  and  gray  matter  ; 
the  white  striae  are  composed  largely  of  fibres  derived  from  the 
optic  tracts,  the  gray  zones  probably  also  receiving  collateral  con- 
nections from  the  retina.  The  nerve-cells  of  the  gray  matter,  many 
of  which  are  large  and  deeply  pigmented,  send  axis-cylinder  processes 
as  far  as,  possibly,  the  occipital  cortex.  The  mesial  or  inner 
geniculate  bodies  contain  numerous  nerve-cells  of  medium  size, 
together  with  fibres  seemingly  connected  with  the  mesial  root  of  the 
optic  tract  ;  an  intimate  relation  between  these  bodies  and  the  optic 
fibres,  however,  is  questionable. 

The  masses  of  gray  matter  forming  the  structures  along  the  fore 
part  and  the  floor  of  the  third  ventricle,  including  the  lamina 
cinerea,  tuber  cinereum,  infundibulum,  and  posterior  per- 
forated space,  contain  scattered  ganglion-cells,  together  with 
certain  special  bundles  of  nerve-fibres.  The  corpora  mam- 
millaria  are  composed  of  bundles  of  fibres,  and  contain  gray  nuclei 
within  the  superficial  layer  of  white  matter.  The  hollow  coni- 
cal infundibulum  bears  at  its  lower  extremity  the  pituitary  body, 
with  part  of  which  the  funnel-shaped  extension  of  the  cerebral  vesicle 
is  at  one  time  continuous. 


THE   OLFACTORY   LOBE. 

The  structures  described  in  human  anatomy  as  the  olfactory 
nerves  represent  the  rudimentary  olfactory  lobes,  which  in  many 
of  the  lower  animals  constitute  conspicuous  divisions  of  the  brain. 
This  lobe,  as  found  in  man,  comprises  three  parts, — the  tuber  olfac- 
torium,  the  tractus  olfactorius,  and  the  bulbus  olfactorius. 

The  tuber  olfactorium  does  not  here  call  for  attention,  since  its 
adequate  consideration  lies  without  the  purpose  of  these  pages. 

The  tractus  olfactorius,  on  transverse  section,  exhibits  zones 
of  white  and  gray  matter,  together  with  a  central  flattened  area 
of  neuroglia,  indicating  the  position  of  the  obliterated  lumen, 
which  in  the  embryonic  condition  temporarily,  and  in  the  lower 
animals  permanently,  existed  as  a  continuation  of  the  ventricular 
cavity. 

The  gray  substance  is  richest  in  the  dorsal  part  of  the  tract, 
where  it  forms  an  oval  area  surrounded  by  medullated  nerve-fibres  ; 
these  latter  become  continuous  at  the  lateral  angles  with  the  thick 
medullary  fibre-layer  occupying  the  ventral  zone,  the  juncture 
between  the  two  being  marked  by  a  thickening  of  the  medullary 


324 


NORMAL  HISTOLOGY. 


stratum.  Enclosed  within  the  continuous  ring  of  fibres  lies  the  flat- 
tened gelatinous  or  neuroglia  zone  corresponding  to  the  area  of 
the  obliterated  former  lumen  of  the  tract.  Outside  the  fibre-layer, 
a  sheet  of  gray  substance,  extremely  thin  on  the  ventral  surface, 

represents  the  cortex 


Fig.  348. 


3  ^^^i^i&fSiyil 


£&.-• 


Section  of  portion  of  human  olfactory  bulb :  1,  3,  bundles  of 
transversely-cut  nerve-fibres,  enclosing  central  neuroglia  (2)  ; 
4,  5,  6,  granule  layer ;  7,  zone  of  olfactory  glomeruli  {g) ;  8, 
layer  of  olfactory  nerve-fibres,  bundles  (0)  of  which  pass  to 
olfactory  mucous  membrane.     (After  Henle.) 


of  the  convolution. 

The  Bulbus  Olfac- 
torius.  While  the 
layers  present  in  the 
tract  are  continued  into 
the  terminal  olfactory 
bulb,  the  greater  devel- 
opment of  the  ventral 
zone  considerably  mod- 
ifies the  structure  of  this 
division  of  the  olfactory 
lobe.  In  the  bulb  the 
area  of  the  obliter- 
ated ventricular  cav- 
ity lies  eccentrically, 
closely  approaching  the 
dorsal  surface,  from 
which  it  is  separated  by 
a  thin  layer  of  fibres 
and  the  greatly  attenu- 
ated gray  cortical 
stratum,  here  reduced 
to  a  delicate  lamina. 
The  ventral  layers, 
on  the  contrary,  are 
nerve-fibres  which  pass 


greatly  developed,    and  culminate  in   the 

through  the  cribriform  plate  as  the  true  olfactory  nerves. 

In  transverse  section,  the  dorsal  portion  of  the  olfactory  bulb  is 
occupied  by  (i)  the  central  neuroglia,  the  atrophic  field  representing 
the  obliterated  lumen  of  the  lobe  ;  this  area  is  enclosed  by  (2)  the 
flattened  ring  of  medullary  substance,  consisting  of  closely- 
placed  bundles  of  longitudinal  nerve-fibres. 

Next  the  ventral  portion  of  the  medullary  ring  lies  (3)  the  stratum 
granulosum,  a  thick  zone  of  gray  matter  containing  numerous 
ganglion-cells  of  different  size.  Some  of  these  are  small  irregular 
elements,  and  immediately  beneath  the  ring  constitute  a  dense  aggre- 
gation. The  most  conspicuous  elements  of  the  gray  matter,  how- 
ever, are  the  large  pyramidal  or  mitral  cells  (30-50  u)  which 
occupy  the  deeper  parts  of  this  stratum. 


THE    CENTRAL    NERVOUS    SYSTEM. 


325 


The  protoplasmic  processes  of  the  mitral  cells  extend  ven- 
trally  and  terminate  in  two  ways,  as  shown  by  the  investiga- 
tions of  Golgi,  Ramon  y  Cajal,  Retzius,  and  others.  In  addition 
to  the  apical  fibrils,  lateral  processes  diverge  and  terminate  in 
free     arborizations 


Fig.  349. 


within  the  ventral  por- 
tion of  the  gray  mat- 
ter. The  apical 
processes  take  part 
in  the  formation  of 
(4)  the  olfactory 
glomeruli,  peculiar 
masses  (.05-3  mm.) 
composed  of  dense 
interlacements  of 
terminal  ramifications 
of  the  apical  pro- 
cesses sent  out  by 
the  mitral  cells  and  of 
the  olfactory  nerve - 
fi  lam  e  nt  s.  The 
axis-cylinder  pro- 
cesses of  the  mitral 
cells  pass  dorsally  and 
become  continuous 
with  nerve-fibres  of 
the  medullary  ring  ; 
during  their  course 
they  give  off  recur- 
rent collateral 
branches. 

The  layer  of  olfac- 
tory glomeruli  is 
followed  by  (5)  the 
stratum  of  olfac- 
tory nerve -fibres. 
These  are  non-med- 
ullated,  and  arise  in 

the  olfactory  cells  of  the  Schneiderian  membrane,  whence  they 
pass  into  the  cranium  and  end  in  arborizations  included  within 
the  olfactory  glomeruli,  to  whose  formation  they  thus  con- 
tribute. The  exterior  of  the  olfactory  bulb  is  invested  by  a  layer 
of  pia  broken  by  the  passage  of  the  nerve-fibres  and  the  blood- 
vessels. 


Sagittal  section  of  part  of  olfactory  bulb  of  young  rabbit,  stained 
after  Golgi's  silver  method  :  tn,  mitral  ganglion-cells  from  which 
pass  axis  cylinder  processes  {a),  sending  off  recurrent  collateral 
branches  (r)  and  protoplasmic  processes  (j>) ;  h,  horizontal 
processes  extending  tangentially  ;  gt  glomeruli  from  whose  com- 
plex of  nerve-fibrils  pass  olfactory  nerves  (o)  ;  n,  filaments  de- 
scending to  mucous  membrane.     (After  Retzius.) 


326 


NORMAL    HISTOLOGY. 


THE   WHITE   MATTER   OF   THE    CEREBRUM. 

The  parts  of  the  cerebrum  thus  far  considered  have  included  areas 
composed  chiefly  of  gray  substance ;  it  remains  to  notice  briefly  the 
complex  mass  of  nerve-fibres  forming  the  conspicuous  medulla. 
It  has  already  been  stated  that  the  nerve-fibres  constituting  these 
central  masses  are  mostly  medullated,  but  devoid  of  a  neuri- 
lemma ;  the  fibres  vary  in  diameter,  those  pursuing  an  extended 
course  connected  with  the  large  motor  cells  possessing,  in  general, 
a  greater  diameter  than  those  related  with  sensory  areas. 

The  accurate  determination  of  the  arrangement  of  the  various 
nerve-tracts  included  within  the  medulla  is  a  labor  of  great  difficulty 
and  one  still  far  from  satisfactory  completion  ;  notwithstanding  the 
great  advances  made  in  this  field  of  investigation  since  the  introduc- 

Fig.  350. 


LoBe 


Diagram  of  association  fibres  of  cerebrum:  s,  short  fibres  connecting  adjacent  gyri ;  f.l.s., 
superior  longitudinal, /.Li.,  inferior  longitudinal,  f.u  ,  uncinate,  and_/T/.,  perpendicular  fasciculus; 
ei,  cingulum  ;  fo,  fornix  ;  fi,  fimbria  ;  v.d'A  .,  bundle  of  Vicq  d'Azyr.     (After  Schaefer-Meynert.) 


tion  of  the  improved  methods  (Weigert's)  for  tracing  medullated 
fibres,  much  remains  to  be  learned  regarding  the  course  and  the 
distribution  of  many  tracts  connecting  the  central  nuclei  with  the 
cerebral  cortex. 

The  great  mass  of  the  cerebral  medulla  is  composed  of  fibre- 
tracts,  which  may  be  grouped  into  three  systems  : 

1.  The  association  fibres,  connecting  parts  of  the  same  hemi- 
sphere. 

2.  The  commissural  fibres,  uniting  parts  of  the  two  hemispheres, 


THE  CENTRAL   NERVOUS   SYSTEM.  ,27 

and  represented  by  the  fibres  of  the  corpus  callosum  and  of  the 
anterior  commissure. 

3.  The  projection  fibres,  streaming-  from  the  entrance  of  the 
brain-stalks,  or  cerebral  peduncles,  secondarily  also  from  the  basal 
nuclei,  to  spread  out  in  the  various  parts  of  the  cerebral  cortex  and 
thus  constitute  the  conspicuous  corona  radiata. 

The  association  fibres  consist  of  bundles  of  various  length, 
which  unite  :  (a)  adjoining  convolutions,  passing  from  the  medulla 
of  one,  beneath  the  intervening  fissure,  into  the  white  matter  of  the 
neighboring  gyrus;  (&)  adjacent  convolutions,  but  not  immedi- 
ately adjoining ;  (c)  more  distant  parts  of  the  hemisphere. 

The  most  important  of  these  longer  tracts  are  : 

1.  The  fasciculus  uncinatus,  connecting  the  inferior  frontal 
convolution  with  the  uncinate  gyrus  of  the  temporal  lobe. 

2.  The  fasciculus  longitudinalis  inferior,  connecting  the  an- 
terior part  of  the  temporal  with  the  apex  of  the  occipital  lobe. 

3.  The  fasciculus  longitudinalis  superior,  connecting  the 
middle  of  the  frontal  partly  with  the  occipital  and  partly  with  the 
apex  of  the  temporal  lobe. 

4.  The  cingulum,  extending  along  the  corpus  callosum  within 
the  cingulate  convolution. 

5.  The  fasciculus  perpendicularis,  connecting  the  inferior 
parietal  with  the  fusiform  lobe. 

6.  The  fornix,  connecting  the  uncinate  process  of  the  hippo- 
campal  convolution  with  the  thalamus  by  means  of  the  continuations 
effected  by  the  fimbria  behind  and  the  bundle  of  Vicq  d'Azyr,  from 
the  mammillary  body  to  the  thalamus,  in  front. 

The  majority  of  the  commissural  fibres,  which  connect  similar 
regions  on  the  two  sides,  take  part  in  the  formation  of  the  great 
transverse  bridge,  the  corpus  callosum ;  these  fibres,  the  pro- 
longations of  the  axis-cylinder  processes  of  the  cortical  ganglion- 
cells  or  of  the  collateral  processes  derived  from  the  projection  fibres, 
pass  to  all  parts  of  the  cerebral  surface,  with  the  exception,  probably, 
of  the  anterior  portions  of  the  temporal  lobes  and  the  olfactory 
tracts,  which  parts  are  connected  by  the  fibres  of  the  anterior 
commissure.  On  either  side  of  the  closely-packed  bundles  con- 
stituting the  immediate  bridge  the  fibres  spread  out  in  a  fan-like 
course  to  reach  their  destination. 

The  projection  or  peduncular  fibres  include  many  of  the  most 
important  tracts  by  means  of  which  communication  between  the 
presiding  cortical  centres  and  the  more  deeply  lying  nuclei  and  paths 
is  established.  The  bundles  of  the  crusta  on  reaching  the  sub- 
thalamic region  become  continuous  with  the  internal  capsule  and 
spread  out  into  the  conspicuous  corona  radiata.     The  fibres  which 


-,2g  NORMAL    HISTOLOGY. 

gain  the  cortex,  however,  do  not  correspond  exactly  with  those  enter- 
ing the  cerebrum  as  the  peduncular  bundles,  since  some  of  the  latter 
are  deflected  and  pass  to  the  caudate  and  the  lenticular  nucleus  from 
the  internal  capsule  ;  on  the  other  hand,  the  peripherally-streaming 
bundles  are  augmented  by  fibres  which  come  from  the  thalamus  and 
the  subthalamic  region.  The  peduncular  tracts  continued  to  the 
cortex  consist  principally  of  {a)  the  pyramidal  fibres,  (b)  the  fibres 
from  lateral  tracts,  sensory  paths  to  the  temporo-occipital  (?)  region, 
and  (V)  the  fibres  from  the  pontine  nuclei  and  the  cerebellum. 

The  tracts  of  the  tegmentum  largely  contain  fibres  related  to 
the  connections  of  the  thalami,  the  cerebellum,  and  the  corpora 
quadrigemina  ;  regarding  the  exact  course  and  communications  of 
these  bundles  much  still  remains  to  be  determined. 

Two  small  but  remarkable  organs,  the  pituitary  and  the  pineal 
body,  are  closely  associated  in  their  genetic  relations  with  the  cere- 
brum, since  the  first  of  these  bodies  originates  partly  and  the  second 
entirely  as  a  diverticulum  from  the  cavity  of  the  primary  inter-brain. 

THE    PITUITARY   BODY. 

The  pituitary  body,  or  hypophysis  cerebri,  consists  of  two 
portions,  the  large  anterior  oral  and  the  small  posterior  cerebral 
division.  These  are  entirely  distinct  both  in  structure  and  in  de- 
velopment, since  the  anterior  lobe  is  derived  as  a  diverticulum 
from  the  primitive  oral  cavity,  and,  as  such,  is  lined  with  the  oral 
ectoderm,  while  the  posterior  lobe  descends  as  an  outgrowth 

from   the   floor  of   the   primary   inter- 
Fig.  351.  brain,  its  stalk  remaining  as  the  infun- 

dibulum. 

In  the  embryo  temporarily,  and  in 
many  lower  vertebrates  permanently,  the 
tissues  composing  the  posterior  lobe 
assume  a  distinctly  nervous  type  ;  in  the 
higher  animals,  however,  this  character  is 
lost,  the  lobe  remaining  small  and  rudi- 
mentary and  its  cavity  undergoing  obliter- 
ation ;  the  primary  nervous  character  of 
the  cerebral  lobe  disappears  as  the  in- 
growth of  the  connective  tissue  and 
the  blood-vessels  takes  place.  The  re- 
mains of  the  immature  nervous  elements 
are  sometimes  recognized  in  the  branched 
and  spindle  pigmented  cells  found  in  this  part  of  the  pituitary 
body,  as  well  as  in  the  partially-preserved  cavity  lined  with  ciliated 
columnar  cells. 


Section  of  human  pituitary  body  : 
C,  portion  of  posterior  or  nervous 
lobe ;  P,  portion  of  anterior  or 
glandular  lobe,  consisting  of  tubular 
acini  (a) ;  s,  connective-tissue  septa  ; 
v,  blood-vessels. 


THE  CENTRAL   NERVOUS   SYSTEM.  ,2Q 

The  anterior  lobe,  larger  and  darker  than  the  preceding,  for 
some  time  remains  connected  by  its  tubular  ectodermic  stalk 
with  the  primitive  oral  cavity  ;  later  the  tube  becomes  atrophic  and 
finally  disappears,  the  end  of  the  oral  diverticulum  then  lying  iso- 
lated and  separated  from  the  buccal  cavity.  The  single  primary 
tube  undergoes  repeated  division,  producing  compartments  which 
appear  in  the  adult  organ  as  slightly  convoluted  tubular  acini.  The 
tubules  are  held  together  by  vascular  connective  tissue,  and  contain 
polyhedral  epithelial  cells,  with  spherical  or  oval  nuclei,  irregularly 
disposed  and  often  almost  filling  the  alveoli ;  the  lumina  of  the  tubules 
are  sometimes  occupied  by  colloid  masses  resembling  those  of  the 
thyroid  gland. 

THE    PINEAL    BODY. 

The  pineal  body,  epiphysis,  or  conarium,  since  the  compara- 
tively recent  investigations  of  Spencer  and  of  de  Graaf,  although 
known  and  described  previously  for  centuries,  is  now  regarded  as  a 
rudimentary  sense-organ.  These  investigators  independently 
demonstrated  that  the  structure 

seen   in   man    and    the    higher  Fig.  352. 

animals  is  the  rudiment  of  what 
was  a  functionating  sense-organ 
in  the  extinct  reptiles,  and  even 
in  certain  living  members  of  the 
same  class  strongly  resembles 
an  imperfect  invertebrate  eye  in 
its  early  embryonal  condition. 
In  the  light  of  our  present 
knowledge,  therefore,  this  pe- 
culiar body  must  be  looked 
upon  as  representing  an  im- 
perfect organ  of  special  sense, 
whether  as  an  additional  visual 
structure — the  "pineal  eye" 
— or  as  an  organ  for  the  percep- 
tion of  warmth  still  remains  to 
be  determined. 

In  man  and  other  mammals 
the  pineal  body,  instead  of  oc- 
cupying its  morphologically 
normal  position  on  the  superior 
surface  of  the  brain,  is  covered 
over  by  the  greatly  developed 
cerebral  hemispheres,  so  that  its  final  position  is  well  towards  the 
base  of  the  encephalon.     The  organ  at  no  time  in  the  higher  ani- 


Sagittal  section  through  part  of  head  of  lizard 
embryo,  showing  so-called  pineal  eye:  P, special- 
ized isolated  extremity  of  pineal  diverticulum  from 
brain-vesicle  (Z?) ;  b,  c,  so-called  retinal  and  len- 
ticular areas  of  its  walls  ;  a,  ectoderm  ;  d,  remains 
of  diverticulum  undergoing  division  into  tubules 
(d') ;  f,  blood-vessels  ;  e,  mesodermic  tissue. 


330 


NORMAL    HISTOLOGY. 


Fig.  353- 


mals  assumes  the  characters  of  a  sense-organ  to  the  extent  seen  in 
the  lower  types. 

The  adult  human  pineal  body  is  composed  of  a  number  of  tu- 
bular compartments   or  alveoli,  which 
are  separated  by  septa  of  connective  tissue 
and  lined  by  polyhedral  epithelial  cells  ; 
in  many  places  the  tubules  are  almost  oc- 
cluded by  epithelium,  together  with  aggre- 
gations of  gritty  calcareous  matter,  the 
so-called  "  brain-sand."     The  brain-sand, 
or  acervulus  cerebri,  consists  of  irregu- 
larly round  mammillated  or  mulberry-form 
concretions  of  variable  size,  composed  of 
animal  matter  combined  with  earthy  salts 
(calcium    carbonate    and    phosphate   with 
magnesium    and    ammonium    phosphate). 
These  deposits  are  not  limited  to  the  in- 
terior of  the  pineal  body,  but  are  encountered  on  its  exterior  and  on 
the  peduncles,  as  well  as  in  the  choroid  plexus  and  in  other  parts  of 
the  brain-membranes  ;  the  concretions  occur 
at  all  ages,  even  before  birth,  and  within  the 
perfectly  normal  organ. 

Other  bodies,  the  corpora  amylacea, 
occur  as  round  discoidal  masses,  and  exhibit 
a  distinct  concentric  striation  ;  they  are 
regarded  as  amylaceous  in  nature,  since  they 
respond  to  the  tests  for  such  substances, 
staining  violet  with  iodine  and  sulphuric  acid.  These  bodies  are 
almost  constant  within  both  the  gray  and  the  white  matter  consti- 
tuting the  walls  of  many  parts  of  the  brain-cavities  ;  the  olfactory  tract 
is  a  particularly  favorite  situation,  along  this  region  the  amylaceous 
corpuscles  occurring  with  especial  profusion. 


Section  of  human  pineal  bjdy : 
a,  a,  acini  lined  and  partially 
filled  with  epithelium  and  cal- 
careous concretions  (s) ;  /,  inter- 
tubular  fibrous  tissue. 


Fig.  354- 


Corpora  amylacea  from   lateral 
ventricles  of  human  brain. 


THE   SUPRARENAL    BODY. 

The  close  relations  of  this  organ  with  the  nervous  system,  as 
evidenced  by  its  early  history,  the  profusion  of  its  nervous  elements, 
and  the  results  of  pathological  processes,  entitle  the  suprarenal  body 
to  place,  provisionally  at  least,  within  the  present  chapter. 

The  parenchyma  of  the  organ,  composed  of  a  peripheral  zone, 
the  cortex,  and  a  central  area,  the  medulla,  is  invested  by  a  fibrous 
capsule  of  considerable  thickness.  From  this  envelope  numerous 
connective-tissue  septa  penetrate  deeply  into  the  soft  cellular 
substance,  which  is  thus  broken  up  into  cylindrical  masses. 

The  cortex    consists  of  aggregations  of  irregularly  rounded   or 


THE   CENTRAL   NERVOUS   SYSTEM. 


331 


Fig.  355- 


polygonal  cells  (13-17  /->-),  whose  granular  protoplasm  frequently 
contains  fat-particles  in  addition  to  clear  nuclei.  The  arrangement 
of  the  cortical  elements  varies  at  different  levels,  the  resulting 
disposition  giving  rise  to  the  three 
divisions  of  the  cortex  recognized  as 
the  zona  glomerulosa,  the  zona  fas- 
ciculata,  and  the  zona  reticularis. 
The  cells  forming  the  first  of  these  are 
grouped  as  oval  masses,  those  of  the 
middle  layer  are  disposed  as  long 
cylindrical  groups,  and  those  of  the 
third  stratum  are  irregularly  arranged 
as  anastomosing  cords  supported 
within  a  reticulum  of  connective  tissue. 
The  zona  reticularis  is  distinguished 
from  the  other  cortical  layers  by  the 
pigmented  condition  of  its  cells.  The 
various  groups  of  cellular  elements  are 
separated  from  one  another  by  delicate 
fibrous  septa,  continuations  from  the 
outer  connective-tissue  envelope ;  the 
larger  septa  support  the  capillary  net- 
works which  surround  the  groups  of 
cells. 

The  medulla  contains  granular,  fre- 
quently deeply-pigmented,  polygonal 
cells  arranged  as  cords  and  irregular 
net  -  works  within  a  framework  of 
highly  vascular  connective  tissue.  Nu- 
merous ganglion  -  cells  occupy  the 
central  portions  of  the  medulla,  along 
with  a  rich  net-work  of  non-medullated  nerve-fibres  and  the  con- 
spicuous venous  channels. 

The  blood-vessels  of  the  organ  divide  within  the  capsule  into 
numerous  smaller  branches,  which  enter  the  parenchyma  along  the 
fibrous  septa  ;  capillary  net-works  surround  the  cell-groups  of 
both  cortex  and  medulla.  The  veins  of  the  medulla  are  of  large 
size  and  unite  to  form  trunks  which  make  their  exit  at  the  central 
hilus  ;  the  larger  radicles  are  accompanied  by  longitudinal  bundles 
of  non-striped  muscle. 

The  nerves  of  the  suprarenal  body  are  remarkable  for  their  num- 
ber and  size ;  they  bear  the  arteries  company  within  the  septa  to 
reach  the  medulla,  where  they  form  an  intricate  plexus  composed 
chiefly  of  non-medullated  fibres.     Ganglion-cells  occur  along 


Section  of  human  suprarenal  body  : 
a,  fibrous  capsule ;  b,  zona  glomeru- 
losa ;  c,  zona  fasciculata  ;  d,  zona  reti- 
cularis ;  e,  medullary  cords  ;  f,  venous 
channel ;  g,  ganglion-cells. 


532 


NORMAL   HISTOLOGY. 


the  course  of  the  nerve-trunks,  and  are  found  within  the  medulla  in 
considerable  numbers. 

The  lymphatics  are  represented  by  delicate  canals  within  the 
fibrous  septa  which  communicate  with  the  intercellular  clefts  of  both 
cortex  and  medulla  on  the  one  hand,  and  with  the  larger  lymph-ves- 
sels within  the  capsule  on  the  other. 

THE    DEVELOPMENT    OF   THE    NERVOUS   TISSUES. 

The  consideration  of  the  general  changes  involving  the  primary 
neural  tube  and  its  cephalic  expansions,  the  brain-vesicles,  by  which 
are  produced  the  various  portions  of  the  cerebro-spinal  axis,  belongs 
to  embryology,  and  lies  without  our  present  purpose  ;  an  account 
of  the  histogenesis  of  the  nervous  tissues,  however,  is  of  much 
interest  in  indicating  the  true  relations  of  the  structural  components 
of  the  great  nervous  masses. 

The  essential  parts  of  the  nervous  system,  including  the  nerve- 
cells,  the  nerve-fibres,  and  the  neuroglia,  are  developed  from  the 
ectoderm  alone,  and  all  result  from  the  differentiation  and  speciali- 
zation of  the  walls  of  the  primary  neural  tube. 

This  canal  is  formed  by  the  gradual  closure  of  the  early  furrow, 

the  medullary  groove,  of  the 
Fig.  356.  invaginated  ectoderm  along 

the  dorsal  line  ;  by  the  approxi- 
mation of  the  upper  or  dorsal 
edges  of  the  involution  the 
furrow  is  converted  into  a 
tube,  the  sides,  or  medullary 
plates,  of  the  extreme  ante- 
rior and  posterior  segments  of 
which  are  the  last  to  unite. 
Even  before  the  closure  of  the 
groove  has  been  completed  a 
differentiation  of  two  impor- 
tant regions  is  indicated;  these 
are  the  areas  giving  rise  to  the  segmental  ganglia  and  to  the 
general  axial  nerves. 

The  area  for  the  latter  is  represented  by  the  lining  of  the  neural 
tube,  that  for  the  former  by  the  inconspicuous  cell-groups  lying  on 
either  side  of  the  line  of  closure. 

The  primary  wall  of  the  neural  tube  consists  of  a  single  layer  of 
columnar  epithelial  cells,  whose  nuclei  occupy  the  middle  third  of 
the  elements,  leaving  an  outer  and  an  inner  free  zone  ;  within  the  latter 
appears  very  early  a  second  variety  of  cell,  which  is  distinguished 
by    its  large   spherical    form    and    conspicuous    nucleus.      The 


Section  of  nine-day  rabbit  embryo,  showing  open 
neural  tube  :  e ,  ectoderm  invaginated  and  thickened 
within  neural  canal  («)  ;  m,  mesoderm  ;  b,  body- 
cavity ;  g,  still  open  gut,  lined  by  entoderm  (h). 


Fig.  357. 


THE   CENTRAL   NERVOUS   SYSTEM.  -,  ^5 

round  cells  invading  the  inner  zone  represent  the  ancestors  of  the 
nervous  elements — both  cells  and  fibres — and  are  the  germ-cells, 
while  the  columnar  cells 
produce     the      neuroglia 
and     are     the     spongio- 
blasts. 

The  development  of 
the  nerve-cells  proceeds 
from  the  germ-cells, 
which,  as  shown  by  the 
karyokinetic  figures  within 
their  nuclei,  undergo  ac- 
tive proliferation,  the 
resulting  progeny  being 
the  neuroblasts,  from 
which  the  nerve  -  cells 
are  directly  derived.     The 

germ-cells   are  confined  to   the   zone   next  the   brain-cavity, 
which  thus  indicates  the  position  of  greatest  formative  energy. 

The  neuroblasts  at  first  occupy  the  innermost  zone,  next  the 
cerebral  cavity,  but  soon  migrate  towards  the  outer  boundary  of  the 
wall,  at  the  same  time  becoming  pyriform  and  elongated.  The 
young  nerve-cells  for  a  long  time  possess  but  a  single  process, 


Section  of  ten-day  rabbit  embryo,  showing  closed  neural 
tube  :  n,  neural  canal ;  s,  area  from  which  segmental  gan- 
glia develop ;  m,  mesodermic  tissue ;  g,  gut-tube ;  v,  v, 
primitive  aorta  ;  /,  pleuro-pericardial  cavity. 


Fig.  358. 


Primary  wall  of  neural  tube  composed 
of  single  layer  of  epithelium  (a)  ;  6,  5, 
germ-cells  occupying  inner  zone.  (After 
//is.) 


Portion  of  inner  zone  of  wall  of  neural 
tube  in  which  round  germ-cells  (ft,  n) 
and  partially-developed  neuroblasts  («', 
n')  lie  among  the  surrounding  spongio- 
blasts.    (After  His.) 


which  grows  out  to  become  a  nerve-fibre,  and  therefore  represents 
the   axis-cylinder,  or  nerve-process ;    the  protoplasmic   pro- 


334 


NORMAL   HISTOLOGY. 


cesses,  whose  ramifications  later  present  such  striking  pictures,  are 
subsequently  acquired,  after  the  lapse  of  considerable  time. 


Portion  of  wall  of  neural  tube,  exhibiting  germ-cells  (g)  among  the  differentiating 
spongioblasts  (s).     (After  Hzs.) 

The  development  of  the  neuroglia  depends  upon  the  special- 
ization of  the  columnar  elements,  the 
spongioblasts.  The  epithelial  cells 
elongate,  their  protoplasm  at  the  same 
time  undergoing  vacuolation  and 
partial  absorption,  resulting  in  the 
production  of  an  elongated  framework 
of  connected  slender  trabeculae.  The 
extremities  of  the  changed  epithelial 
elements,  or  spongioblasts,  greatly  dif- 
fer ;  the  inner  ends  of  the  cells  extend 
to  the  inner  boundary,  where  they  are 
united  to  form  a  continuous  sheet,  the 
membrana  limitans  interna,  and  the 
outer  processes  break  up  into  irregular 
branches,  which  ultimately  form  a  close 
reticulum.  The  early  spongioblasts  ex- 
tend the  entire  thickness  of  the  neural 
wall,  but  with  the  subsequent  increase  in 
this  structure  their  attachments  become 
broken,  the  spongioblasts  then  lying  free 
among  the  surrounding  nervous  elements. 
The  general  growth  of  the  tissues  is  ac- 
companied by  great  extension  and  sub- 
division of  the  neuroglia  fibres,  which 
eventually  become  the  nucleated  masses  of  fine,  bristle-like  processes 


Spongioblasts  from  neural  tube ; 
their  expanded  upper  ends  unite  to 
constitute  the  internal  limiting  mem- 
brane next  the  brain-cavity ;  their 
outer  ends  break  up  into  reticulum. 
(After  His.) 


THE   CENTRAL    NERVOUS    SYSTEM.  53$ 

constituting  Deiters's  or  spider  cells.  The  spongioblasts  im- 
mediately around  persistent  parts  of  the  neural  canal  retain  their 
inner  connection  and  form  a  continuous  layer  of  lining  elements, 
which  later  constitute  the  ciliated  columnar  epithelium  of  the 
ependyma. 

The  development  of  the  nerve-fibres  includes  the  origin  of 
two  sets  of  primary  fibres — those  derived  from  the  nerve -cells 
of  the  medullary  tube  and  those  growing  out  from  the  cells  of 
the  ganglia.  All  nerve-fibres  are  formed  as  the  direct  exten- 
sions and  continuations  of  the  processes  of  the  neuroblasts. 
In  the  case  of  those  proceeding  from  the  neural  canal  the  fibres 
grow  peripherally  and  the  cells  remain  attached  to  their  central 
ends,  thus  early  establishing  the  relations  afterwards  existing  between 
the  motor  cells  and  the  fibres  ;  those  originating  from  the  ganglia, 
on  the  other  hand,  grow  in  two  directions,  towards  the  periphery 
and  towards  the  nervous  axis,  representing  the  sensory  paths. 

The  early  nerve-fibres  consist  for  some  time  of  the  axis-cylin- 
der alone,  the  neurilemma  and  the  medullary  substance  being  not 
only  much  later  acquisitions  but  also  contributions  from  the  meso- 
derm. The  neurilemma  first  envelops  the  ectodermic  axis-cylin- 
der as  a  delicate  sheath,  and  subsequently  within  this  envelope  the 
myeline  of  the  white  matter  of  Schwann  is  deposited.  The  ap- 
pearance of  this  coat  is  often  very  late,  and  takes  place  at  different 
times  for  the  various  tracts  of  nerve-fibres,  although  the  period  at 
which  the  several  groups  acquire  their  medullary  substance  is  con- 
stant and  definite  for  each  set.  The  young  fibres  soon  collect 
into  groups,  which  represent  the  early  nerve-trunks,  whose  further 
growth  proceeds  in  a  straight  path  corresponding  with  the  general 
direction  of  the  component  axis-cylinders  ;  a  course  once  established 
is  maintained  until  arrested  by  some  obstacle  or  modified  by  changes 
in  the  position  of  the  parts  with  which  the  nerve  has  formed  attach- 
ments. The  terminations  of  the  growing  nerves  are  abrupt,  the 
finer  ramifications  appearing  only  after  the  trunk  has  undergone 
repeated  branchings. 


336 


NORMAL   HISTOLOGY. 


CHAPTER    XVII. 

THE    EYE   AND    ITS   APPENDAGES. 
THE   EYEBALL. 

The  bulbus  oculi  consists  of  three  coats:  i,  the  external 
fibrous  tunic,  comprising  the  sclerotic  and  the  cornea,  upon  which 
devolves  the  maintenance  of  the  form  of  the  organ  ;  2,  the  middle 
vascular  tunic,  made  up  of  the  choroid,  the  ciliary  body,  and  the 
iris,  to  which  the  principal  vascular  supply  of  the  eye  is  distributed  ; 
3,  the  inner  nervous  tunic,  the  retina,  which  receives  the  termi- 
nal expansion  of  the  optic  nerve  and  contains  the  specialized  neuro- 
epithelium  concerned  in  the  perception  of  the  visual  stimulus.  The 
aqueous  humor,  the  crystalline  lens,  and  the  vitreous  body  are  en- 
closed by  these  coats,  and  represent  the  refractive  media  of  the  eye. 

Referred  to  their  embryonic  origin,  the  several  parts  of  the  eye 
may  be  grouped  under  two  headings, — those  developed  from  the 
ectoderm  and  those  derived  from  the  mesoderm.  The  mem- 
bers of  the  first  group  may  be  subdivided  into  (a)  structures  derived 
directly  from  the  ectoderm,  including  the  lens  and  its  anterior  epi- 
thelium, and  the  epithelium  of  the  cornea  and  of  the  adjacent  scleral 
surface,  and  (b)  structures  derived  secondarily  from  the  ectoderm 
through  the  optic  vesicles  protruded  from  the  involuted  ectoderm  of 
the  cerebral  vesicles  ;  to  this  class  belong  the  primary  retinal  tissues, 
including  the  pigment-layer,  as  well  as  the  atrophic  retinal  layers 
continued  over  the  posterior  surface  of  the  ciliary  body  and  the  iris. 
All  other  parts  of  the  eyeball,  comprising  the  remaining  portions 
of  the  sclera,  the  cornea,  the  iris,  the  ciliary  body,  the  choroid,  and 
the  vitreous  body,  as  well  as  the  connective-tissue  ingrowths  of  the 
retina,  are  developed  from  the  mesoderm. 

the  cornea. 

The  cornea  consists  of  five  layers  : 

1.  The  a?iterior  epithelium. 

2.  The  anterior  limiting  membrane. 

3.  The  substance  proper. 

4.  The  posterior  limiting  membrane. 

5.  The  posterior  endothelium. 

The  anterior  epithelium,  the  only  part  of  the  cornea  derived 
from  the  ectoderm,  all  others  being  mesodermic,  is  stratified  squa- 


THE    EYE   AND    ITS   APPENDAGES. 


337 


mous  in  character ;  it  is  thinnest  over  the  central  part  of  the  cornea, 
where  its  six  to  eight  layers  of  cells  together  measure  about  45  fi,  at 
the  periphery  reaching  almost  double  that  thickness.  The  deepest 
cells  are  columnar  in  form  with  their  outer  ends  somewhat  rounded 
off,  while  their  bases  are  slightly  expanded  and  conform  to  the  sur- 
face of  the  basement-membrane  upon  which  they  rest.  Succeeding 
the  deepest  layer  the  elements  become 
broader  and  polyhedral,  many  pos- 
sessing the  protoplasmic  threads  char- 
acteristic of  prickle-cells.  The  su- 
perficial strata  are  composed  of 
flattened  cells  which  lie  parallel  to 
the  surface  and  contain  oval  nuclei. 

The  anterior  limiting  mem- 
brane, membrane  of  Bowman,  or 
lamina  elastica  anterior,  corre- 
sponds to  a  highly-developed  base- 
ment-membrane, being  continuous 
with  the  tissue  of  the  substantia  pro- 
pria, of  which  it  is  a  special  conden- 
sation. The  structure  is  especially 
conspicuous  in  the  human  cornea, 
where  it  appears  as  a  seemingly  ho- 
mogeneous glassy  layer,  about  20  p. 
in  thickness  in  the  middle  of  the  cor- 
nea, gradually  diminishing  towards  the 
periphery.  The  resolution  of  this 
lamina  into  delicate  connective- 
tissue  fibrillae  after  treatment  with 
suitable  reagents  demonstrates  its 
true  nature  as  a  specialized  portion  of 
the  substantia  propria. 

The  fibrous  stroma,  or  the  sub- 
stantia propria,  constitutes  the  chief 
bulk  of  the  cornea,  and  is  made  up 
of  parallel  lamellae  composed  of  in- 
terlacing bundles  of  fibrous  connective  tissue.  The  exact  num- 
ber of  corneal  lamella?  is  inconstant,  since  this  depends  upon  the 
extent  to  which  the  artificial  separation  of  the  tissue  is  carried.  The 
interlacing  bundles  of  the  white  fibrous  tissue  composing  the 
lamellae  are  united  by  the  interfibrillar  cement  substance,  and  cross 
one  another  obliquely  at  various  angles,  the  adjacent  bundles  being 
intimately  united  by  bands,  the  fibrae  arcuatae,  which  pass  from 
one  bundle  to  the  other  ;  the  arcuate  fibres  are  especially  prominent 


Section  of  human  cornea :  a,  anterior 
epithelium  ;  c,  anterior  limiting  membrane  ; 
b,  b,  fibrous  substantia  propria,  containing 
corneal  corpuscles  (_/")  lying  within  cor- 
neal spaces ;  d,  posterior  limiting  mem- 
brane ;  e,  endothelium  lining  anterior 
chamber. 


3^g  NORMAL   HISTOLOGY. 

in  the  anterior  lamellae.      The  substantia  propria  resembles  the 
matrix  of  cartilage  in  yielding  chondrin  on  boiling,  therein  differ- 

Fig.  363. 


Interlacing  bundles  of  fibrous  tissue  constituting  substantia  propria  from  cornea  of  ox  ;  interstitial 
injection  with  silver  nitrate. 

ing  from  the  sclera,  which,  like  the  usual  connective  tissues,  produces 
gelatin. 

The  cellular  elements,  the  corneal  corpuscles,  are  plate-like 
or  stellate  connective-tissue  cells,  whose  branched  processes  unite 

Fig.  364. 


Corneal  corpuscles  from  calf;  gold  preparation. 


with  those  from  adjacent  cells  to  constitute  a  protoplasmic  reticu- 
lum throughout  the  tissue.  The  corneal  cells  and  their  processes 
lie  within  a  system  of  intercommunicating  lymphatic  spaces 


THE   EYE   AND   ITS   APPENDAGES.  ^^g 

hollowed  out  within  the  cement-substance,  which  consists  of  the 
large  lacunae  or  corneal  spaces  between  the  lamellae  and  the 
small  canaliculi  extending  from  the  former  as  fine  branching  tubes. 
The  corneal  corpuscles  are  usually  applied  to  one  wall  of  the  spaces, 
which    they    by   no    means 

completely  fill,   while  their  Fig-  365- 

processes  extend  within  the  XJ/^  .  f J  >■-» 

branching     canaliculi.       In  '.'"''«-,',i    .',  \  •"     : 

addition  to  the  corneal  cor-  ..;;^  '■■'%<■ :/;^:'-  ■."-;,-' 

puscles,  wandering  cells,       f'v,  ,,f?€; 
together  with   the  tissue- 
juices,  occupy  the  spaces 
and  canaliculi. 

The  posterior  limiting 
membrane,      membrane 
of  Descemet,  or  poste- 
rior   elastic    lamina,  ap- 
pears as  a  clear  homoge- 
neous band  at  the  inner  boundary  of  the  cornea,  sharply  defined 
from  the  deepest  layers  of  the  substantia  propria  and  clothed  on  its 
inner  surface  by  endothelium.    The  membrane  differs  from  the  cor- 
responding anterior  lamella  in  its  less  intimate  attachment  with  the 


Corneal  spaces  from  calf;  silver  preparation. 


Fig.  366. 


w 


A 


Corneal  spaces  from  calf;  exhibited  spaces  in  positive  picture  after  interstitial  silver  injection. 

fibrous  stratum  and  in  possessing  the  greatest  thickness  (10-12  /x)  at 
the  periphery.  After  prolonged  maceration  or  treatment  with  suit- 
able reagents  the  resistant  lamina  is  separable  into  a  number  of  thin 


34Q 


NORMAL   HISTOLOGY. 


homogeneous  layers,  which  sometimes  exhibit  a  delicate  longi- 
tudinal striation. 

The  posterior  corneal  endothelium,  or  endothelium  of  Des- 
cemet's  membrane,  consists  of  a  single  layer  of  regular  poly- 
hedral plates,  whose  oval  nuclei  project  slightly  beyond  the  bodies 
of  the  cells. 

Blood-vessels  are  absent  in  the  cornea,  except  within  a  narrow 
zone,  about  i  mm.  in  width,  at  the  limbus  or  margin  ;  in  the  foetus, 
however,  the  vessels  extend  well  towards  the  centre  and  form  the 
precorneal  capillary  net-work. 

The  lymphatics  of  the  cornea  are  represented  by  the  system  of 
intercommunicating  spaces  and  canaliculi ;  these  clefts  open 
into  lymphatic  radicles  at  the  periphery,  which,  in  turn,  communi- 
cate with  the  larger  anterior  lymphatic  vessels.  Perineurial  lymph- 
channels  enclose  the  larger  nerve-trunks,  which  they  accompany 
for  a  variable  distance  into  the  corneal  tissue ;  these  lymphatic 
channels  communicate  directly  with  the  corneal  spaces  at  frequent 
intervals. 

The  nerves  of  the  cornea  are  very  numerous,  and  are  distributed 
largely  within  the  anterior  layers.  They  enter  at  the  corneal  limbus 
as  some  sixty  radially-disposed  twigs,  each  of  which  includes  from 

Fig.  367. 


Subbasilar  plexus  of  corneal  nerves  from  rabbit ;  gold  preparation. 


three  to  twelve  fibres;  the  latter  almost  at  once,  within  .5  mm.  of 
the  limbus,  become  non-medullated. 

Within  the  substantia  propria  the  nerve-fibres  form  a  coarse 
ground-plexus  at  a  level  corresponding  to  about  the  middle  third 
of  the  corneal  tissue ;  from  this  net-work  twigs  are  sparingly  given 
off  to  supply  the  deepest  layers,  while  others  pass  towards  the 


THE    EYE   AND    ITS   APPENDAGES.  o^i 

anterior  lamellae,  in  which  they  form  net-works.  Immediately 
beneath  the  anterior  elastic  membrane  the  smaller  fibres  form  the 
dense  subbasilar  plexus,  while  under  the  epithelium  the  finest 
fibrillar  constitute  the  subepithelial  plexus,  from  which  delicate 
naked  axis-cylinders  ascend  and  enter  the  epithelium,  to  end  between 
the  cells  as  the  intra-epithelial  plexus. 

THE    SCLERA. 

The  sclera  is  composed  of  the  same  elements  as  is  the  substantia 
propria  of  the  cornea,  but  they  are  less  regularly  disposed  and  lack 
the  remarkable  transparency  of  the  latter. 

The  ground-substance  is  made  up  of  interlacing  bundles  of 
gelatin-yielding  fibrous  tissue  mingled  with  elastic  fibres  ;  the  fibrous 
bundles  are  arranged  as  two  principal  sets,  those  extending  longi- 
tudinally or  meridionally  and  those  running  transversely  or 
equatorially. 

The  interfascicular  interspaces  are  occupied  by  the  stellate 
connective-tissue  plates,  which  correspond  closely  to  the  corneal 
corpuscles ;  in  addition,  a  few  small  wandering  cells  are  usually 
present.     The  sclerotic  and  cho- 
roid coats  are  united  by  a  layer  Fig.  368. 
of  loose  connective   tissue,    the      r^^^^i^^S^-^gE^^^^^g^, 


lamina    suprachoroidea,    the  ps;;.~<  _  ^'*v ""       /s     •    "'"f|J:° 
extensive  interfascicular  clefts  of '      .€ %V   *''''•        '  *'W&  i 


■-■■Z  :■■■.:■■ 


which    form    part    of    the    sub-      gr~-    i-*~~-  '■    ■> -•--'-  — --'- ^ 

scleral  lymph-space.  ^^^^4^'^^^^^^P'  ^ 


Sj»5* 


/SJSSg 


The  suprachoroidal  tissue 
consists  of  many  imperfect  la- 
mellae composed  of  a  fibro- 
elastic  groundwork  support- 
ing irregular  groups  of  flattened 
endothelioid  connective-tissue 
plates  ;  the  broad  trabecular  join 
one  another  at  various  angles, 
and  include  the  imperfectly  sep- 
arated compartments  of  the  gen-  ||jl  v, H ;-:v"  ;J5ll||y  Spb^i  jS3m 
eral  lymph-space.  The  larger  p ^^^^^^5^^^^^~Sffi-^fr -- ■ 
partitions  convey  the  numerous  ^    ^—       "^~ 

vascular  and  nervous  trunks  in       Section  of  bumfn  Tba"  tak en  ™id,way  be" 

tween  equator  and  posterior  pole  :  S,  sclera  ;  p, 
their  COUrSe  tO  and  from  the  lamina  fusca  and  lamina  suprachoroidea  ;  P,  peri- 
rVinrniH  scleral  tissue ;  C,  choroid ;  R,  retina  with  its  layers 

indicated  by  figures. 

The   deeply-pigmented  tissue 
of  the  innermost  layer  of  the  sclera,  next  the  subscleral  space, 
constitutes  the  lamina  fusca,  and  is  covered  with  the  endothe- 


342 


NORMAL    HISTOLOGY. 


lial  lining  of  the  lymph-cavity.  The  outer  surface  of  the  sclera 
throughout  a  large  part  of  its  extent  takes  part  in  bounding  the 
episcleral  space,  where  it  is  likewise  covered  with  endothelium. 

The  blood-vessels  distributed  to  the  tissue  of  the  sclerotic  coat 
are  meagre,  although  the  tunic  is  pierced  by  numerous  trunks  related 
with  the  supply  of  the  underlying  parts ;  such  small  vessels  as  are 
present  break  up  into  capillaries  passing  among  the  bundles  of  fibrous 
tissue. 

The  lymphatics  are  represented  by  the  intercommunicating  cell- 
spaces  which  connect  with  the  larger  lymph-cavities. 

The  nerves  terminating  within  the  sclera  constitute  fine  twigs 
given  off  from  the  larger  trunks  passing  between  the  sclerotic  and 
choroid  coats  ;  they  break  up  into  fibrillar  which  end  as  naked  axis- 
cylinders  between  the  bundles  as  an  interfascicular  plexus. 

THE    CHOROID. 

The  choroid  consists  of  a  connective-tissue  stroma  supporting 
numerous  blood-vessels.  Dependent  largely  upon  the  size  and 
arrangement  of  the  blood-vessels,  certain  layers  are  distinguished, 
these  being,  from  without  inward  : 

i.    The  layer  of  choroidal  stroma  containing  large  blood-vessels. 

2.  The  layer  of  dense  capillary  net-works — the  choriocapillaris. 

3.  The  homogeneous  glassy  lamina,  or  vitreous  membrane. 

The  stroma-layer,  with  its  large  blood-vessels,  constitutes  the 
greater  part  of  the  choroid.     Within  a  supporting  tissue  made  up 

Fig.  369. 


Section  of  human  choroid  :  a,  retinal  pigment  adhering  to  vitreous  mem- 
brane (b)  ;  c,  capillary  layer,  or  choriocapillaris ;  d,  e,  large  blood-vessels 
of  stroma-layer  (_/") ;  g,  lamina  suprachoroidea ;  h,  tissue  of  sclera. 

of  closely  united  connective-tissue  lamellae,  elastic  fibres,  and 
branched  pigmented  cells,  the  freely  branching  arterial  and  venous 
trunks  take  their  course,  appearing  as  lighter-colored  channels  within 
the  darker  surrounding  matrix.     The  blood-vessels  and  the  stroma 


THE    EYE   AND    ITS   APPENDAGES. 


343 


are  so  intimately  united  that  they  constitute  a  layer  of  considerable 
consistence.  The  largest  vessels  occupy  the  most  superficial 
part  of  the  stratum,  those  next  in  size  the  middle,  while  the  smallest 
approach  the  capillary  zone.  The  most  conspicuous  of  the  large 
superficial  blood-channels  are  the  four  venae  vorticosae,  with  their 
whorls  of  tributaries.  These  veins  occupy  positions  around  the 
equator  at  points  about  equidistant,  towards  which  the  smaller  vessels 
converge  from  all  directions,  returning  the  blood  not  only  from  the 
choroid  but  also  from  the  ciliary  body  and  the  iris.  The  veins  of 
the  choroid  are  often  surrounded  by  perivascular  lymph-sheaths. 
Many  of  the  larger  arteries,  in  addition  to  the  well-marked  circu- 
larly-disposed muscle  with  which  they  are  provided,  are  accompanied 
by  external  longitudinal  bundles  of  involuntary  muscle. 

The  innermost  part  of  the  stroma-layer,  next  the  choriocapil- 
laris,  forms  a  narrow  stratum  (10  fi  in  width)  which  is  devoid,  or 
nearly  so,  of  pigment,  and  constitutes  the  boundary  zone.  In 
the  eyes  of  many  animals  (horse,  cow,  sheep)  this  layer  possesses 
wavy  bundles  of  connective  tissue,  to  whose  peculiar  arrangement 

Fig.  370. 


Human  choroid  seen  from  its  inner  surface,  exhibiting  surface  view  of  cap- 
illary net-work,  or  choriocapillaris  (c,  c)  ;  b,  b,  large  blood-vessels  of  stroma- 
layer  beneath  ;  a,  a,  intervening  stroma-tissue. 

is  due  the  metallic  reflex  sometimes  seen  from  such  eyes  ;  this 
shining  structure  is  known  as  the  tapetum  fibrosum,  as  distinguished 
from  the  iridescent  tapetum  cellulosum  of  the  carnivora  which  is 
dependent  upon  the  presence  of  several  layers  of  plate-like  cells 
containing  innumerable  small  crystals. 

The  capillary  layer,  or  choriocapillaris,  consists  ol  a  narrow 


^  NORMAL    HISTOLOGY. 

zone,  about  10  ft  in  width,  at  the  inner  part  of  the  choroid,  composed 
of  a  structureless,  apparently  homogeneous,  matrix,  in  which  lie 
embedded  the  close  capillary  net-works  derived  from  the  terminal 
branches  of  the  short  ciliary  arteries. 

The  vitreous  lamina,  or  glassy  membrane,  forms  the  most 
internal  layer  of  the  choroid  and  supports  the  retinal  pigment.  The 
membrane  presents  a  delicate  homogeneous  stratum  (2  fi  in  thick- 
ness), ordinarily  without  appreciable  structure,  and  is  very  intimately 
associated  with  the  adjoining  layer  of  the  choroid  ;  to  its  inner  sur- 
face patches  of  retinal  pigment  frequently  adhere  on  removal  of 
the  retina. 

The  nerves  of  the  choroid,  non-medullated  fibres  distributed  to 
the  blood-vessels,  are  derived  from  the  plexus  formed  within  the 
suprachoroidal  tissue  by  branches  given  off  from  the  long  and  short 
ciliary  nerves  in  their  transit  through  the  subscleral  space. 


^-^•^ 


THE   CILIARY    BODY. 

This  structure  includes  that  portion  of  the  uveal  tract  situated  be- 
tween the  termination  of  the  choriocapillaris,  opposite  the  ora 
serrata  behind  and  the  ciliary  or  outer  margin  of  the  iris  in  front. 
Within  this  important  territory  three  areas  may  be  distinguished  : 

1,  the  ciliary  ring;   2,  the  cili- 
Fig.  371.  ary   processes;    3,    the    ciliary 

muscle. 

The  ciliary  ring,  or  orbicu- 
lus  ciliaris,  is  a  circular  tract 
about  4  mm.  in  breadth,  situated 
immediately  in  front  of  the  ora 
serrata  and  extending  to  the 
posterior  ends  of  the  ciliary  pro- 
cesses. This  zone  differs  from 
the  choroid  in  the  absence  of 
the  choriocapillaris  and  in  the 
presence  of  muscular  tissue 
prolonged  from  the  mass  of  the 
ciliary  muscle  ;  the  character  of 
the  stroma  also  changes,  its  bulk 
being  here  made  up  of  fibrous 
connective  bundles  instead  of  elastic  lamella;. 

The  ciliary  processes  consist  of  an  annular  series  of  some 
seventy  prominent  radial  vascular  folds  which  project  from  the 
inner  surface  of  the  ciliary  body  and  arise  from  the  confluence  of 
several  of  the  low  ridges  on  the  ciliary  ring  ;  after  attaining  a  height 
of  about  1  mm. ,  they  abruptly  end  at  the  base  of  the  iris,  sinking 


Section  of  human  ciliary  processes  :  /,  in- 
terstitial connective-tissue  S'roma,  covered  by 
retinal  layers  (R)  ;  /,  o,  inner  clear  and  outer 
pigmented  layers  of  cells  ;  /,  fibrous  tissue  of 
processes. 


THE    EYE   AND    ITS   APPENDAGES.  24- 

to  the  level  of  the  underlying  ciliary  muscle.  The  stroma  of  the 
processes  is  a  continuation  of  the  connective  tissue  of  the  orbicular 
zone,  this  layer  being  the  true  prolongation  of  the  choroid,  since  the 
muscular  tissue  must  be  regarded  as  an  intercalation  between  the 
sclerotic  and  choroid  coats.  The  vitreous  lamina  is  continued  as  a 
delicate  homogenous  membrane,  3  to  4  ,u  in  thickness,  over  the  inner 
surface  of  the  ciliary  processes.  Inside  this  layer  the  internal  face  of 
this  entire  region,  including  the  ciliary  ring  and  the  ciliary  body,  as 
well  as  the  iris,  is  covered  by  the  deeply-pigmented  rudimentary 
layers  of  the  pars  ciliaris  retinae,  consisting  of  an  inner  small 
row  of  tall  columnar  elements  and  an  outer  sheet  of  low  pig- 
mented epithelium.  Since  these  layers  represent  the  rudimentary 
folded  anterior  laminae  of  the  ectodermic  optic  vesicle,  the  ciliary 
processes  and  the  iris  consist  of  two  genetically  distinct  parts,  the 

Fig.  372. 


Section  through  ciliary  region  of  human  eye  :  A,  cornea;  a,  b,  c,  its  epithelium,  substantia  propria, 
and  endothelium ;  C,  scleral  conjunctiva,  terminating  at  d;  B,  sclera,-  e,  sclero-comeal  juncture  ;  D, 
iris  ;  E,  ciliary  body  covered  by  pigment-layer,  /;  k,  fibrous  stroma  of  ciliary  processes ;  f,  bands  of 
pectinate  ligament ;  g ,  spaces  of  Fontana  ;  s,  canal  of  Schlemm ;  v,  venous  channels  ;  wz,  n,  o, 
meridional,  radial,  and  circular  (Miiller's)  fibres  of  ciliary  muscle ;  r,  subscleral  space  bridged  by 
fibrous  bands. 


mesodermal  connective-tissue  stroma,  containing  blood-vessels 
and  muscle-fibres,  and  the  inner  deeply-pigmented  ectodermal 
stratum. 


^45  NORMAL   HISTOLOGY. 

The  ciliary  muscle  presents  a  conspicuous  thickening  for  about 
i  mm.,  which  extends  from  the  orbicular  zone  to  the  base  of  the  iris 
and  bears  on  its  inner  surface  the  connective-tissue  stroma  of  the 
ciliary  processes  and  the  orbicular  ring.  In  meridional  sections  its 
mass  appears  as  a  triangular  area,  the  cross-section  of  a  three-sided 
annular  band  of  muscle  entirely  encircling  the  eyeball.  The 
triangle  thus  formed  closely  approximates  a  right  angle  whose  sides 
are  unequal ;  the  shorter  anterior  side  extends  from  the  sclero- 
corneal  juncture  towards  the  ciliary  processes,  and  the  longer  inner 
border  is  prolonged  to  meet  the  outer  side  or  hypothenuse  at  an 
acute  angle  at  the  anterior  border  of  the  choroid. 

The  mass  of  the  ciliary  muscle  consists  of  interlacing  bundles 
of  involuntary  muscle,  the  interspaces  between  which  are  filled 
by  connective  tissue.  The  muscular  fasciculi  are  arranged  as 
three  sets,  the  meridional,  the  radial,  and  the  circular.  The 
meridional  fibres  lie  generally  parallel  to  the  sclera,  and  form  a 
compact  layer  attached  in  front  at  the  sclero-corneal  junction, 
near  the  anterior  margin  of  Schlemm's  canal,  and  behind  at  the 
fore  margin  of  the  choroid,  where,  in  common  with  many  of  the 
radial  fibres,  it  finds  insertion  ;  in  recognition  of  this  attachment,  the 
meridional  and  radial  fibres  were  named  the  tensor  choroideae. 
The  radial  bundles  spread  out  fan-like  from  their  anterior  attach- 
ment, the  most  external  fibres  running  nearly  parallel  to  the  meridio- 
nal bundles,  with  which  they  become  continuous,  while  the  anterior 
pass  off  at  a  considerable  angle.  The  circular  fibres,  the  ring-mus- 
cle of  Miiller,  constitute  a  distinct  group  of  equatorially-disposed 
bundles,  which  occupy  the  internal  angle  of  the  ciliary  muscle  and 
extend  at  right  angles  to  the  preceding  bundles. 

The  blood-vessels  of  the  ciliary  body  are  especially  concerned 
in  supplying  the  ciliary  muscle,  to  which  minute  arterial  twigs 
pass  from  the  imperfect  vascular  circle  lying  behind  the  arterial 
circle  of  the  iris. 

The  numerous  nerves  of  the  ciliary  muscle  are  derivatives  of  the 
ciliary  trunks,  which  on  entering  the  muscle  form  a  plexus  within 
its  substance ;  from  this  plexus  fibres  pass  internally  to  the  iris, 
outwardly  to  the  cornea,  while  others  are  distributed  to  the 
ciliary  muscle  itself.  Small  ganglion-cells  also  occur,  singly  or 
in  small  groups. 

THE    IRIS. 

The  iris  constitutes  the  anterior  segment  of  the  uveal  tract,  and 
consists  of  a  principal  stroma-layer  covered  in  front  by  a  reflection 
of  the  corneal  endothelium  and  behind  by  the  continuation  of  the 
deeply-pigmented  rudimentary  retinal  layers — the  pars  iridica 


THE   EYE  AND   ITS   APPENDAGES. 


347 


retina.     The  various  components  of  the  iris  and  their  morphological 
relations  may  be  grouped  as  follows  : 


5.   Pigment-layer,  < 


>  of  OPTIC  VESICLE. 


Anterior  endothelium. 

Anterior  boundary  layer,  ~\  Continuation  of  the  tissues  of  the 
Vascular  stroma-layer,  >  uveal  tract  proper,  constituting 
Posterior  boundary  layer,  )      the  stroma-zone. 

a.  Anterior  layer  of  pig-  ~\ 
merited     spindle  -  cells 
representing      outer 
layer 

b.  Posterior  layer  of  pig- 
mented polygoyial  cells 
representing  inner 
layer 

The  anterior  endothelium  consists  of  a  single  layer  of  thin  nu- 
cleated polygonal  plates,  the  direct  prolongation  of  the  corneal 
endothelium.  The  protoplasm  of  the  cells  is  finely  granular,  but 
always  free  from  pigment. 

The  anterior  boundary  layer  is  formed  by  modification  of  the 
foremost  stratum  of  the  iris-stroma ;  the  connective  tissue  consti- 

Fig.  373. 


Section  through  part  of  iris  and  lens,  from  human  eye  :  /,  iris  ;  a,  anterior 
endothelium ;  b,  anterior  boundary  layer ;  c,  vascular  stroma ;  d,  posterior 
boundary  layer;  e,  pigment-layer  continued  as  far  as  g  on  pupillary  margin 
(P) ;  _/",  cut  circular  muscle-bundles  of  sphincter;  L,  surface  of  crystalline 
lens ;  h,  anterior  lens  capsule,  with  anterior  epithelium  beneath  ;  i,  tissue  of 
lens. 

tuting  this  layer  resembles  the  reticular  tissue  of  lymphatic  struct- 
ures, comprising  several  layers  of  net-works  within  the  interspaces 
of  which  lie  lymphoid  cells  in  greater  or  less  profusion. 

The  vascular  stroma  constitutes  the  chief  mass  of  the  iris,  and, 
in  addition  to  its  numerous  blood-vessels,  contains  involuntary 


,4g  NORMAL   HISTOLOGY. 

muscle  and  nerves.  The  stroma  consists  of  loose  spongy  re- 
ticular connective  tissue  greatly  strengthened  by  the  radially- 
disposed  blood-vessels  and  nerves,  around  which  the  delicate 
stroma  forms  ensheathing  masses  of  considerable  density.  The 
clefts  situated  between  these  adventitious  sheaths  and  the  included 
vessels  and  nerves  form  a  system  of  lymphatic  channels  through- 
out the  iris  which  communicate  with  the  anterior  chamber  through 
the  lymph-spaces  at  the  irido-corneal  angle. 

The  arteries  of  the  iris  spring  from  the  anterior  part  of  the  cir- 

culus  arteriosus   iridis  major, 
Fig.  374.  situated  at  the  ciliary  border,  and 

pass  towards  the  centre  of  the  iris 
as  radially-disposed,  freely-anasto- 
mosing twigs  ;  about  1  mm.  from 
the  inner  edge  of  the  iris  these 
vessels  unite  to  form  a  second 
delicate  vascular  ring,  the  circulus 
arteriosus  iridis  minor,  which 
marks  the  division  of  the  iris  into 
its  pupillary  and  ciliary  zones, 
which  are  respectively  1  mm.  and 
3-4  mm.  in  breadth.  From  this 
circle  the  arterioles  continue  their 
course  towards  the  pupillary  bor- 
der, and  end  in  the  capillary  net- 
work distributed  to  the  sphincter 
muscle.  Capillary  reticula  exist  also  within  the  anterior  and  pos- 
terior layers  of  the  stroma.  All  the  capillaries  are  tributary  to  the 
radiating  veins  which  pass  to  the  ciliary  border,  where  they  join  those 
of  the  ciliary  processes  and  finally  empty  into  the  radicles  forming 
the  venae  vorticosae. 

Bundles  of  involuntary  muscle  occupy  the  pupillary  border  and 
the  posterior  zone  of  the  stroma-layer  ;  these  are  arranged  as  two 
sets, — the  annular  bundles  encircling  the  pupillary  margin  of  the  iris 
and  constituting  the  sphincter  of  the  pupil,  a  muscular  zone  about 
1  mm.  in  width,  and  the  few  scattered  radially-disposed  bundles 
extending  from  the  pupil  towards  the  ciliary  margin  and  forming  an 
incomplete,  by  no  means  continuous,  layer,  the  dilator  pupillae. 

The  posterior  boundary  layer,  or  vitreous  lamella,  appears 
as  a  glassy  structureless  membrane,  about  2  /x  in  thickness,  which 
stretches  over  the  posterior  surface  of  the  stroma  and  supports 
the  pigment-layer :  in  the  nature  of  its  substance  this  structure 
closely  approaches  elastic  tissue. 

The  pigment-layer,  or  pars  iridica  retinae,  is  usually  so  densely 


Injected  iris  from  eye  of  dog  :  P,  pupillary 
margin,  around  which  capillary  net-work  is 
formed  by  vessels  proceeding  from  lesser  ar- 
terial circle. 


THE   EYE  AND   ITS   APPENDAGES.  «*g 

packed  with  deeply-colored  particles  that  its  real  constitution  is 
masked.  This  stratum  is  composed  of  two  layers,  an  anterior  and 
a  posterior.  The  anterior  or  outer  layer  is  formed  of  radially- 
arranged  spindle-cells  which  pass  without  interruption  from  the 
ciliary  border  of  the  iris  to  the  pupillary  margin  ;  at  the  ciliary 
border  the  cells  change  their  form  and  arrangement,  becoming 
polyhedral  and  circularly  disposed  and  continuous  with  the  low 
pigmented  elements  constituting  the  corresponding  layer  of  the 
ciliary  processes. 

The  posterior  layer  presents  a  thicker  zone  (30-35  /1)  of  pig- 
mented cells,  in  which  the  colored  particles  are  so  densely  packed 
that  the  cell-boundaries  and  the  nuclei  are  completely  masked,  the 
entire  layer  appearing  as  one  continuous  mass  of  pigment. 

The  pigment-layer  covers  the  entire  pupillary  margin,  and 
often  ends  as  a  somewhat  thickened  free  edge  slightly  in  advance  of 
the  plane  of  the  iris  ;  at  this  border,  which  represents  the  free 
anterior  lip  of  the  embryonic  secondary  optic  cup,  both  strata 
of  the  pigment-layer  become  continuous. 

The  posterior  surface  of  the  pigment-layer  is  covered  by  a  very 
delicate  cuticular  membrane,  the  membrana  limitans  iridis, 
which  is  continued  from  the  similar  structure  extending  over  the  cili- 
ary processes  ;  it  appears  first  at  the  ora  serrata  as  a  new  formation, 
since  a  true  membrana  limitans  interna,  in  the  sense  of  a  distinct 
cuticle,  does  not  exist  over  the  retina  proper. 

The  marked  variation  in  the  color  of  the  iris  is  largely  dependent 
on  the  amount  and  position  of  its  pigment.  In  blue  eyes  the 
stroma  of  the  iris  is  entirely  free  from  pigment,  the  latter  being 
confined  to  the  posterior  pigment-layer,  from  which  position  it  is 
seen  through  the  superimposed  iridal  strata.  With  the  darker  color 
of  the  iris  its  stroma-cells  also  acquire  pigment ;  in  light  gray 
eyes  this  is  small  in  amount,  in  brown  eyes  greater,  while  in  the 
darkest  eyes  the  colored  particles  are  very  numerous  and  sometimes 
appear  as  almost  continuous  pigmented  areas  ;  in  albino  eyes,  on  the 
other  hand,  even  the  retinal  portion  of  the  iris  is  devoid  of  pigment. 

The  nerves  of  the  iris,  derived  from  the  intra-muscular  ciliary 
plexus,  enter  the  more  superficial  part  of  the  stroma-layer  as  med- 
ullated  fibres.  Within  the  iris  the  nerve-fibres  soon  lose  their  med- 
ullary sheath  and  form  one  or  two  irregular  net-works,  the  most 
constant  of  which  is  a  circular  plexus  in  the  vicinity  of  the  sphinc- 
ter muscle  ;  from  this  net-work  pale  fibres  are  distributed  to  the 
substance  of  the  latter  muscle.  The  principal  plexus  lies  anterior 
to  the  plane  of  the  chief  vascular  net-work,  the  posterior  zone  of  the 
iris  being  poorly  supplied  with  nerves. 

The  irido-corneal  angle,  marking  the  junction  of  the  cornea,  the 


or0  NORMAL   HISTOLOGY. 

sclera,  the  iris,  and  the  ciliary  muscle,  constitutes  one  of  the  most 
important  regions  in  the  eye,  not  only  with  regard  to  its  anatomical 
details,  bfit  also  in  view  of  its  practical  clinical  significance. 

As  already  described,  the  substantia  propria  of  the  cornea  passes 
directly  into  the  ground-substance  of  the  sclera  ;  in  consequence  of 
the  rearrangement  of  the  tissue-elements  of  the  two  structures 
taking  place  soonest  in  the  superficial  planes  of  the  cornea,  the  line 
of  transformation  becomes  oblique,  thereby  producing  an  apparent 
overlapping  of  the  sclera  in  front,  and  a  corresponding  extension  of 
the  cornea  behind. 

The  posterior  elastic  membrane,  on  reaching  the  corneal  mar- 
gin, splits  up  into  a  number  of  stiff  homogeneous  fibres,  many  of 
which  become  attached  to  the  base  of  the  iris  and  constitute  the  liga- 
mentum  pectinatum  iridis.  By  the  union  of  the  processes  from 
the  iris  and  Descemet's  membrane  with  the  elastic  fibres  derived 
from  the  anterior  attachment  of  the  ciliary  muscle  and  a  few  bands 
from  the  sclera,  a  reticulum  of  thin  trabeculae  is  formed,  which 
occupies  the  angle  between  the  cornea  and  the  iris.  This  spongy 
tissue  constitutes  an  annular  mass  enclosing  a  system  of  intercom- 
municating cavities,  the  spaces  of  Fontana.     These  clefts,  lined 

Fig-  375- 


Section  through  irido-corneal  angle  of  human  eye,  highly  magnified  :  a,  substantia  propria 
of  cornea ;  b,  posterior  limiting  membrane,  splitting  at  corneal  margin  into  delicate  lamellae 
(d)  ;  c,  endothelium  continued  over  iris  (i)  ;  /,  elastic  lamellae  separating  Schlemm's  canal  (5) 
from  spaces  of  Fontana  (s,  s)  and  giving  attachment  to  fibres  of  ciliary  muscle  (h). 

by  an  imperfect  layer  of  endothelium,  are  more  conspicuous  in  the 
eyes  of  some  of  the  lower  animals  (horse,  ox,  pig,  sheep),  where 
they  are  far  better  developed  than  in  man. 

Within  the  sclera,  close  to  its  inner  border  and  the  corneal  juncture, 


THE   EYE  AND   ITS   APPENDAGES.  ^Cj 

lies  a  flattened  annular  channel,  the  canal  of  Schlemm  ;  the 
inner  wall  of  this  canal  is  formed  by  intersecting  delicate  lamellae 
whose  loose  disposition  suggests  an  incomplete  isolation  of  the 
channel  from  the  adjacent  spaces  of  Fontana.  The  nature  of  the 
canal  of  Schlemm,  whether  a  venous  or  a  lymphatic  channel,  has  long 
been  a  subject  of  discussion  ;  the  weight  of  evidence  warrants  re- 
garding it  as  a  venous  canal,  between  which  and  the  lymph-clefts 
represented  by  the  spaces  of  Fontana  free  communication  un- 
doubtedly exists. 

THE   RETINA. 

The  inner  nervous  tunic  of  the  eyeball  includes  the  retina  alone, 
which  extends  from  the  optic  entrance  throughout  the  posterior  seg- 
ment of  the  ball  and  as  far  forward  as  the  pupillary  margin  of  the 
iris.  This  extensive  tract,  corresponding  in  its  morphological  limits 
to  the  secondary  optic  vesicle,  falls  into  three  divisions  :  (i)  the 
pars  optica  retinae,  including  the  entire  posterior  segment  and  end- 
ing at  the  ora  serrata;  (2)  the  pars  ciliaris  retinae,  covering  the 
posterior  surface  of  the  ciliary  zone  and  processes  and  extending  from 
the  ora  serrata  to  the  base  of  the  iris  ;  and  (3)  the  pars  iridica  re- 
tinae, passing  over  the  posterior  surface  of  the  iris  from  the  base  to 
the  anterior  edge  of  the  pupil,  where  it  terminates  as  a  slightly-thick- 
ened margin,  which  corresponds  to  the  free  lip  of  the  double-layered 
optic  cup. 

The  retina  proper,  or  pars  optica  retinae,  consists  of  an  inner 
and  an  outer  lamina,  which  correspond  to  the  inner  and  outer  layers 
of  the  optic  vesicle  ;  the  outer  lamina  includes  the  pigment-layer 
alone,  while  the  inner  lamina  embraces  the  remaining  layers  of  the 
retina.  The  inner  lamina  permits  further  subdivision  of  its  structures 
into  the  neuro-epithelial  and  the  cerebral  layer.  The  relations 
of  these  divisions  to  the  individual  retinal  layers  may  be  expressed  : 

I.  Outer  layer  of  optic  In-  ..  i  i     r>-  ,  ? 

.     J        J     r        t  Pigment-laver.  A.  Pigment  layer. 

vesicle.  ) 

r  Layer  of  rods  and  cones ;     ]     n    »r  ,-,,    ,-   , 

\        J  '      I    B.  Neuro-epithelial 

I  Limiting  membrane  ;  layer. 

I  Outer  nuclear  layer  ;  ^ 

II.  Inner  layer  of  optic  J  Outer  reticular  layer ;  -, 
vesicle.  Inner  nuclear  layer  ; 

Inner  reticular  layer  ;  J.  C.   Cerebral  layer. 

Ganglion-cell-layer ; 
Nerve-fibre-layer.  J 

The  retinal  structures  consist  of  two  parts, — the  nervous  ele- 
ments and  the  supporting  neuroglia.  The  supporting  tissues 
contribute  a  considerable  part  of  the  entire  retina,  but  differ  in  their 
amount  in  the  several  layers.  The  most  conspicuous  constituents 
of  the  supporting  framework  are  long  neuroglia-fibres,  the  radial 


,f2  NORMAL    HISTOLOGY. 

fibres  of  Miiller,  which  extend  through  the  entire  thickness  of 
the  retina.  The  expanded  inner  ends  of  the  supporting  fibres  are 
so  closely  applied  that  they  produce  a  seemingly  continuous  mem- 
brane, the  so-called  membrana  limitans  interna.  The  radial 
fibres  rapidly  diminish  in  diameter  beyond  their  bases,  and  are  con- 
tinued as  narrow  irregular  stalks  giving  off  lateral  branches  in  pro- 
fusion to  the  reticular  layers  ;  within  the  inner  nuclear  layer  each 
fibre  presents  an  irregular  nucleated  enlargement,  and  gives  off 
lateral  processes  for  the  support  of  the  nervous  elements  of  the  inner 
nuclear  layer,  as  well  as  to  each  of  the  succeeding  layers.      At  the 

Fig.  376. 


Diagram  illustrating  the  relation  of  the  retinal  elements.  A,  layer  of  rods  and  cones;  B,  limitans 
externa ;  C,  outer  nuclear  layer;  E,  outer  reticular  layer  (between  the  two  (D)  Henle's  fibre-layer)  ; 
F,  inner  nuclear  layer ;  G,  inner  reticular  layer  ;  H,  layer  of  ganglion-cells  ;  /.fibre-layer;  A",  limitans 
interna,  a,  supporting  fibres  of  Miiller;  b,  c,  rod-  and  cone-visual  cells;  d,  bipolars  belonging  to 
rod-cells  ;  e-i,  bipolars  belonging  to  cone-cells  ;  k-m,  horizontal  nerve-cells;  n,  centrifugal  nerve-fibres  ; 
o-t,  ganglion-cells  connected  with  optic  fibres ;  a-e,  spongioblasts  or  amacrines  arranged  in  layers  ; 
4,  i>,  diffuse  amacrines  ;  tj,  nervous  amacrine.     (Kallius  after  Ramon y  Cajal.) 


inner  border  of  the  rods  and  the  cones  the  expanded  ends  of  the  neu- 
roglia-fibres  form  the  external  limiting  membrane,  delicate  pro- 
cesses extending  from  the  latter  between  the  bases  of  the  rods  and  the 
cones,  which  they  surround  and  embrace  as  the  "fibre-crates." 
In  addition  to  the  long  radial  fibres,  richly-branched  neuroglia- 
cells  occur  within  the  outer  reticular  layer  to  the  fibre-complex  of 
which  they  contribute. 

Within  the  meshes  of  the  framework  just  described  the  ner- 
vous elements  of  the  retina  are  distributed  in  a  manner  charac- 
teristic for  each  layer  :  a  brief  consideration  of  these  is  therefore 
necessary. 


THE    EYE   AND    ITS   APPENDAGES. 


353 


The  nerve-fibre-layer  contains  the  continuations  of  the  optic 
fibres  which,  after  having  lost  their  medullary  substance  in  their 
passage  through  the  lamina  cribrosa,  radiate  as  naked  axis-cylin- 
ders to  all  parts  of  the  retina  as  far  as  the  ora  serrata.  The  fibre- 
layer  is  thickest  at  the  edge  of  the  optic  disk  and  thinnest  at  the 
extreme  retinal  periphery.  Sooner  or  later  the  fibres  forsake  their 
peripherally-directed  course,  and,  bending  sharply,  pass  almost  per- 
pendicularly to  the  ganglion- 
layer  and  other  strata.  FlG-  377- 

The  ganglion-cell-layer  m^Mmm^mM 
consists  of  a  single  row  of 
large  multipolar  nerve- 
cells  (15  to  30  (i),  whose 
axis-cylinder  processes 
are  directed  towards  the  fibre- 
layer  ;  their  branched  pro- 
toplasmic processes,  when 
well  developed,  pass  into  the 
inner  reticular  zone,  to  meet 
the  arborizations  of  the  cone- 
bipolars.  The  ganglion- 
cells  in  the  central  part  of 
the  retina  are  densely  packed 
in  the  macula,  constituting 
overlying  rows,  but  towards 
the  periphery  they  are  less 
plentiful,  and  at  the  ora  ser- 
rata infrequent. 

The  inner  reticular  layer 
presents  a  characteristic  retic- 
ulated tissue  composed  of 
neuroglia  net-works  and 
the  rich  arborizations  of  various  nerve-cells  ;  the  processes  origi- 
nate from  both  the  elements  of  the  ganglion-layer  and  the  cells  of 
the  adjacent  nuclear  stratum. 

The  internal  nuclear  layer  includes  a  number  of  distinct  ele- 
ments, and  presents  two  subdivisions  :  (a)  an  inner  layer  of  nerve- 
cells,  the  spongioblasts,  or  amacrines,  and  (&)  an  outer  layer 
of  the  rod-  and  cone-bipolars  forming  the  ganglion  retinae.  The 
"spongioblasts"  are  not  concerned  in  the  production  of  the  sus- 
tentacular  tissue,  as  their  name — given  under  erroneous  ideas  regard- 
ing their  function — would  imply,  but  are  nervous  elements  whose 
branched  protoplasmic  processes  are  resolved  within  the  inner 
reticular  layer  into  arborizations.      The  cone-bipolars  send  their 

23 


y  'v 

Section  of  human  retina :  a,  internal  limiting  mem- 
brane formed  by  apposition  of  expanded  basis  of  Miil- 
ler's  fibres  (y)  ;  b,  fibre-layer;  c,  layer  of  ganglion-cells 
(z) ;  d,  e,  inner  reticular  and  inner  nuclear  layer ;  ft  g; 
outer  reticular  and  outer  nuclear  layer  ;  h,  outer  limit- 
ing membrane  ;  *',  layer  of  rods  and  cones  ;  k,  portion 
of  pigment-layer  ;  v,  x,  blood-vessels. 


254  NORMAL   HISTOLOGY. 

axis-cylinder  processes  into  the  inner  reticular  layer,  to  end  at 
various  levels  in  arborizations  in  relation  with  the  terminal  filaments 
of  the  ganglion-cells  ;  their  protoplasmic  processes  extend  as  far 
as  the  outer  reticular  layer,  where  they  terminate  in  ramifications 
beneath  the  cone-cells.  The  protoplasmic  processes  of  the  rod- 
bipolars  end  beneath  the  rod-cells,  their  axis-cylinder  processes 
penetrating  the  inner  reticular  stratum,  to  end  in  close  relation  with 
the  ganglion-cells. 

The  outer  reticular  layer  appears  as  a  narrow  zone  made  up  of 
an  intricate  net-work  of  fine  fibres  with  sparingly  distributed 
nuclei.  The  fibrillar  are  derived  from  the  neuroglia  and  from  the 
processes  of  nerve-cells,  among  which  are  the  horizontal  cells 
whose  axis-cylinder  processes  extend  horizontally  within  the  layer, 
often  for  considerable  distances,  to  end  beneath  the  visual  cells. 

The  outer  nuclear  layer  and  the  layer  of  rods  and  cones,  the 
remaining  strata  of  the  inner  lamina  of  the  retina,  together  constitute 
the  neuro-epithelium.  Since  the  rods  and  the  cones  and  the  outer 
nuclear  layer  are  parts  of  a  single  lamina  of  tall  neuro-epithelial 
elements,  the  visual  cells,  of  which  they  are  respectively  the  outer 
and  inner  segments,  these  strata  must  be  regarded  as  subdivisions  of 
the  one  broad  zone,  and  not  as  independent  retinal  layers.  The 
outer  and  inner  segments  are  sharply  separated  by  the  intervening 
membrana  limitans,  through  the  openings  in  which  the  rods  and 
the  cones  protrude.  The  constituents  of  the  neuro-epithelium 
are,  therefore,  the  rod-visual  cells  and  the  cone-visual  cells, 
supported  by  the  sustentacular  tissue. 

The  rod-visual  cells  are  composed  of  two  parts,  the  one  situ- 
ated without  the  limitans,  including  the  non-nucleated  and  highly- 
specialized  segments,  the  rods,  and  the  other  within  the  limitans, 
consisting  of  slender  varicose  elements,  the  rod-fibres,  provided 
with  fusiform  enlargements,  the  rod-spherules,  which  contain  the 
nuclei  of  the  visual  cells.  The  rods  are  slender  cylindrical  struct- 
ures, about  60  ix  in  length  and  2  ft  in  breadth,  composed  of  two 
chemically  and  optically  distinct  parts,  the  outer  and  inner  seg- 
ments. 

The  outer  segments  of  the  rods  are  cylindrical,  apparently 
homogeneous,  highly-refracting  bodies,  which,  after  certain  reagents, 
exhibit  a  disposition  to  break  up  into  thin  transverse  disks.  The 
outer  segments  of  the  rods  are  further  distinguished  as  being  the 
exclusive  seat  of  the  peculiar  visual  purple  or  rhodopsin.  The 
inner  segments  of  the  rods  are  slightly  broader  and  less  regularly 
cylindrical,  and  present  a  finely  granular  appearance,  the  parts  of 
the  segments  nearest  the  membrana  limitans  possessing  a  peripheral 
longitudinal  striation. 


THE    EYE   AND    ITS    APPENDAGES.  ,CC 

The  inner  segments  of  the  rod-visual  cells  include  the  rod- 
fibres  and  their  nucleated  expansions,  the  rod-granules.  The 
rod-fibres  are  slender,  greatly  extended,  and  often  varicose,  and 
reach  from  the  membrana  limitans  into  the  external  zone  of  the 
outer  reticular  layer.  Each  rod-fibre  represents  the  greatly  attenu- 
ated protoplasmic  body  of  a  visual  cell,  the  situation  of  whose 
nucleus  is  indicated  by  the  ellipsoidal  enlargement.  These  enlarge- 
ments, the  rod-granules,  vary  in  position,  sometimes  lying  near  the 
outer  end,  at  other  times  close  to  the  middle  or  the  inner  extremity 
of  the  fibres.  The  granules  are  almost  entirely  occupied  by  the 
nuclei  of  the  visual  cells,  which  are  covered  by  an  extremely  thin 
layer  of  the  cell-protoplasm.  The  nuclei  of  the  cells  are  oval  in  form, 
about  6  /J-  in  length,  and  characterized  by  a  remarkable  differentiation 
of  their  substance  into  lighter  and  darker  transverse  bands. 

The  cone-visual  cells  consist  also  of  two  parts,  the  outer  di- 
visions, the  cones,  situated  beyond  the  membrana  limitans,  and  the 
inner  portions  including  the  cone-fibres  and  their  nucleated  cone- 
granules.  The  cones,  like  the  rods,  present  inner  and  outer  seg- 
ments, which  in  physical  and  chemical  properties  resemble  the 
corresponding  parts  of  the  rods  ;  the  cones,  however,  are  little  more 
than  half  (32-36  /*)  the  length  of  the  rods. 

The  inner  segments  of  the  cones  are  much  wider  than  their 
outer  divisions,  and  appear  as  truncated  conical  bodies  whose 
sides  are  not  absolutely  straight,  but  slightly  convex.  The  outer 
part  of  these  segments  is  occupied  by  an  ellipsoidal  group  of  fine 
longitudinal  fibrillae,  the  fibre-body,  which  corresponds  with  the 
similar  structure  sometimes  present  within  the  rods. 

The  inner  segments  of  the  cone-visual  cells,  representing 
the  bodies  of  the  elongated  cells,  include  the  cone-fibres  and  their 
granules.  The  cone-fibres  differ  from  the  rod-fibres  in  being 
broader  at  the  inner  ends  and  more  regular  in  their  general  contour  ; 
the  cone-granules  always  lie,  except  in  the  macular  region,  next  the 
membrana  limitans. 

The  distribution  of  the  two  kinds  of  visual  cells  varies  in  the 
different  retinal  regions  ;  the  arrangement  prevailing  throughout  the 
greater  part  of  the  retina  is  such  that  the  adjacent  cones  are  separated 
by  three  or  four  rods,  the  latter  far  outnumbering  the  cones.  On 
approaching  the  macula  the  number  of  cones  increases,  the 
cones  being  so  closely  placed  that  they  are  separated  by  only  a  single 
row  of  rods ;  within  the  fovea  itself  the  rods  entirely  disappear, 
the  entire  percipient  layer  being  composed  of  cones  alone.  On 
the  other  hand,  towards  the  periphery  the  number  of  these  visual 
cells  diminishes,  and  at  the  ora  serrata  the  cones  are  -widely 
separated,  while  the  relative  number  of  rods  is  very  large.     The 


,-6  NORMAL    HISTOLOGY. 

conclusion  inferable  from  the  distribution  of  these  elements  in  the  hu- 
man retina,  that  the  cones  are  the  essential  perceptive  instruments,  is 
not  applicable  as  a  generalization,  since  in  many  of  the  lower  animals 
the  cones  are  in  the  minority  or  even  entirely  wanting  (hedgehog, 
shark,  sturgeon),  and  the  rods  predominate ;  it  seems,  however, 
probable  that  the  highest  acuity  of  vision  requires  the  presence  of 
cones.  The  entire  number  of  cones  in  the  human  retina  has  been 
computed  at  something  over  three  and  one-half  millions  (Salzer),  while 
the  rods  are  supposed  to  aggregate  one  hundred  and  thirty  millions. 

The  pigment-layer  represents  the  outer  lamina  of  the  embry- 
onal optic  vesicle,  and  consists  of  a  single  layer  of  polyhedral  epi- 
thelial cells  containing  pigment-granules  in  varying  amount.  These 
cells  ( 1 2-1 8  fi)  are  usually  six-sided,  but  may  have  fewer  or  more 
borders ;  the  cells  in  the  vicinity  of  the  ora  serrata  are  of  exception- 
ally large  size  and  dark  color.  The  elements  of  the  pigment-layer 
exhibit  a  differentiation  into  an  outer  zone  next  the  choroid,  free 
from  pigment  and  containing  an  oval  nucleus,  and  an  inner  zone 
loaded  with  pigment-granules. 

The  inner  part  of  the  pigment-cells  includes  protoplasmic  pro- 
cesses directed  towards  the  layer  of  neuro-epithelium,  between 
the  rods  and  the  cones  of  which  they  extend  for  a  variable  distance  ; 
the  depth  to  which  the  pigment-granules  penetrate  along  the  pro- 
cesses between  the  cells  depends  upon  the  influence  of  light,  since 
under  strong  illumination  the  granules  wander  along  the  protoplas- 
mic processes  as  far  as  the  inner  segment  of  the  rods  and  the  cones, 
while  in  eyes  kept  in  the  dark  for  some  time  before  death  the  intercel- 
lular processes  remain  uninvaded. 

The  structural  details  above  described  represent  the  construction 
of  the  retina  throughout  the  greater  part  of  its  extent  :  two  regions, 
however,  present  such  marked  variations  from  the  typical  arrange- 
ment as  to  call  for  brief  special  mention  ;  these  are  the  macula  lutea 
and  the  ora  serrata. 

The  macula  lutea  and  the  contained  fovea  centralis  corre- 
spond to  the  posterior  pole  of  the  visual  axis,  and  are  distin- 
guished physiologically  by  the  acuity  of  vision,  which  here  attains 
its  highest  degree.  The  macula  lutea  is  characterized,  in  addition 
to  its  yellow  color,  by  a  distinct  thickening  of  certain  of  the  retinal 
layers  and  by  the  absence  of  the  rod-visual  cells  within  its  area. 
The  distinctive  color  of  the  macula  depends  upon  the  presence 
of  diffuse  yellowish  pigment  within  the  layers  internal  to  the 
visual  cells,  the  latter  elements  remaining  colorless  ;  in  consequence 
of  this  arrangement  the  fovea,  in  which  the  neuro-epithelium  alone 
exists,  is  devoid  of  pigment,  and  therefore  appears  as  a  light  spot 
within  the  colored  area. 


THE    EYE   AND    ITS   APPENDAGES. 


357 


The  increased  thickness  of 
the  retina  at  the  macular  margin 
depends  almost  entirely  upon  the 
extraordinary  development  of 
the  layer  of  ganglion-cells, 
which  progresses  until  a  stratum 
from  seven  to  nine  cells  deep  re- 
places the  usual  single  row. 

The  fovea,  on  the  other  hand, 
is  produced  by  the  hollowing  out 
of  the  centre  of  the  macula  conse- 
quent upon  the  gradual  thinning 
and  final  suspension  of  the  reti- 
nal layers  lying  internal  to  the 
outer  nuclear  zone  ;  the  centre 
of  the  foveal  depression,  the  fun- 
dus foveae,  thus  is  composed  of 
the  neuro  -  epithelial  and  the 
pigment  layer  alone.  Within 
a  central  area,  about  .5  mm.  in 
diameter,  the  fovea  is  also  devoid 
of  blood-vessels. 

The  ora  serrata  marks  the 
termination  of  the  optical  part 
of  the  retina  and  the  transition 
into  its  anterior  continuations, 
the  pars  ciliaris  and  the  pars 
iridica.  The  ora  is  distinguished, 
in  addition  to  its  irregular  serrated 
border,  by  the  abrupt  diminu- 
tion in  the  thickness  of  the  retina, 
brought  about  by  the  sudden  ter- 
mination at  this  point  of  many  of 
its  layers.  The  regular  diminu- 
tion in  the  retinal  thickness  pro- 
ceeds gradually  from  the  fundus 
towards  the  periphery,  when,  on 
reaching  a  point  near  the  ora  ser- 
rata, many  layers  end  abruptly, 
the  ciliary  continuation  measuring 
only  about  one-third  of  the  thick- 
ness OI    tUe  adjacent  retina.  Diagrammatic  section  of  human  retina  through 

macula  and  fovea:  2,  fibre-layer;  3,  ganglion- 
cells  :  4,  5,  inner  reticular  and  inner  granule  layer;  6,  outer  reticular  layer  followed  by  obliquely-disposed 
elements  of  outer  nuclear  layer  (7,  ya) ;  9,  rods  and  cones  ;  io,  pigment.     (After  Max  Schtdtze.) 


«cg  NORMAL    HISTOLOGY. 

The  nerve-fibre  and  the  ganglion-cell  layer  having  already  ended 
before  reaching  the  ora,  the  sudden  reduction  is  caused  principally 
by  the  abrupt  termination   of  the  two   reticular   strata.     The 


Fig.  379. 


Section  of  human  retina  through  ora  serrata  :  A,  B,  visual  and  ciliary  portion  of 
retina;  a,  vacuoles;  b,  robust  fibres  of  Miiller;  c,  remains  of  nuclear  layers;  d, 
termination  of  supporting  fibres;  e,  transformation  of  inner  nuclear  layer  into  colum- 
nar cells  within  continuation  of  pigment-layer. 

region  of  the  ora  serrata  is  also  noteworthy  on  account  of  the  re- 
markable development  of  the  radial  fibres  of  Miiller,  which 
here  occur  not  only  in  unusual  numbers  but  also  of  exceptional 
strength. 

Beyond  the  ora  serrata  the  retinal  laminae  are  continued  as  the 
pars  ciliaris  and  the  pars  iridica  retinae.  These  prolongations 
consist  of  an  outer  and  an  inner  lamina.  The  outer  layer  is 
the  direct  and  only  slightly  modified  extension  of  the  retinal 
pigment ;  the  inner  lamina,  the  attenuated  representative  of  the 
remaining  retinal  layers,  consists  of  a  single  row  of  slender  colum- 
nar cells,  which  originate  at  the  ora  by  the  transformation  of  the 
elements  of  the  inner  nuclear  layer.  A  delicate  cuticle,  the 
limitans  interna,  extends  over  the  posterior  surface  of  both  the 
ciliary  body  and  the  iris ;  this  membrane  is  a  true  cuticular  for- 
mation, and  begins  at  the  ora  as  a  new  structure  not  present 
within  the  optical  part  of  the  retina. 

THE   OPTIC    NERVE. 

The  optic  nerve  corresponds  to  a  highly-developed  single  fu- 
niculus, enveloped  by  stout  connective-tissue  sheaths,  which  are 
prolongations  of  the  brain-membranes.  Externally  the  optic  nerve 
is  invested  by  a  robust  fibrous  membrane,  the  dural  sheath,  de- 
rived directly  from  the  dura  ;  this  covering  extends  the  entire  length 
of  the  nerve,  and  on  the  entrance  of  the  latter  into  the  eyeball  be- 
comes continuous  with  the  outer  part  of  the  sclera.  The  surface 
of  the  optic  nerve  is  closely  invested  with  the  pial  sheath,  an 
extension  of  the  pia,  while  between  the  latter  and  the  dural  covering 
lies  a  delicate  partition  from  the  arachnoid,  constituting  the  arach- 
noidean  sheath.     The  clefts  included  between  these  sheaths  con- 


THE    EYE   AND    ITS   APPENDAGES. 


359 


stitute  the  subdural  and  the  subarachnoidean  lymph-spaces  of 
the  optic  nerve,  which  communicate  with  the  corresponding  inter- 
cranial  cavities. 

On  reaching  the  eye-  Fig-  380. 

ball  the  tissue  of  the 
dural  sheath  passes 
uninterruptedly  into  the 
outer  two-thirds  of  the 
sclera ;  the  greater  part 
of  the  pial  sheath 
blends  with  the  inner 
third  of  the  sclera,  some 
of  its  fibres,  however, 
joining  the  choroid. 
The  arachnoidean 
sheath  unites  with  the 
dural,  in  consequence 
of  which  arrangement 
the  subdural  and  sub- 
arachnoidal spaces  be- 
c  o  m  e  continuous  at 
their  ocular  extremities. 

The    trunk    of    the 
optic  nerve,  about  3  mm.  in  diameter,  consists  of  a  great  number 
(almost  800)  of  bundles  of  medullated  nerve-fibres  separated  by 
intervening  fibrous  partitions, 
offshoots  from  the  pial  sheath. 
Each     bundle    is    composed 
of   small    medullated    fibres 
(2  p.),  which  are  without  neu- 
rilemma. 

On  reaching  a  level  corre- 
sponding with  the  confluence 
of  the  sheaths  of  the  nerve 
with  the  sclera,  the  optic  fibres 
pass  through  the  sieve  -  like 
lamina  cribrosa  and  lose 
their  medullary  coat,  contin- 
uing to  their  retinal  distribution 
as  naked  axis  -  cylinders. 
Occasionally  the  medullated 
fibres  retain  their  medullary 
substance  after  their  passage 
conditions  presenting  very  striking  ophthalmoscopic  appearances. 


Transverse  section  of  human  optic  nerve :  d,  dural  sheath  : 
a,  arachnoidean  sheath;  p,  pial  sheath;  n,  bundles  of  nerve- 
fibres  separated  by  fibrous  septa  (e). 


Section  of  human  optic  nerve  under  higher  magni- 
fication :  b,  bundles  of  nerve-fibres  enveloped  in  con- 
nective-tissue sheaths  (x ) ;  n,  neuroglia  nuclei ;  jr, 
nuclei  of  interfascicular  connective  tissue  (z);  v,  blood- 
vessels. 

through  the  lamina   cribrosa,   such 


^5o  NORMAL   HISTOLOGY. 

The  lamina  cribrosa  consists  of  five  to  eight  lamellae,  composed 
of  transversely  extending  fibrous  trabecular,  the  direct  pro- 
longations of  the  scleral  tissue.  These  bands  bridge  across  what 
otherwise  would  be  a  canal,  and  unite  in  such  manner  that  the 
openings  occupied  by  the  nerve-bundles  present  less  area  than  the 
intervening  fibrous  tissue.  The  fibrous  lamellae,  additionally  con- 
nected with  one  another  by  vertical  bands,  pass  from  the  margins 
of  the  scleral  ring  to  the  connective  tissue  supporting  the  blood-ves- 
sels within  the  optic  nerve.     The  lamina  cribrosa  marks  the  nar- 

Fig.  382. 


Longitudinal  section  through  optic  entrance  of  human  eye  :  a,  a,  bundles  of  optic  fibres,  which 
spread  over  retina  at  a',  a' ;  b,  layers  of  retina  terminating  at  edge  of  optic  papilla  ;  c,  choroid  ;  d, 
sclera,  continued  across  optic  nerve  as  lamina  cribrosa  ;  e,g,  i,  respectively  pial,  arachnoidean,  and 
dural  sheaths,  including  subdural  and  subarachnoidean  lymph-spaces ;  /,  /',  retinal  vessels  cut  longi- 
tudinally. 


rowest  diameter  of  the  optic  nerve,  the  loss  of  the  medullary- 
substance,  together  with  the  decrease  in  the  neuroglia,  reducing  the 
size  of  the  nerve  about  one-half.  On  arriving  at  the  margin  of  the 
optic  papilla,  the  bundles  of  nerve-fibres  bend  over  its  edges,  con- 
stituting a  thick  layer,  which  rapidly  thins  away  during  its  radial 
distribution  over  the  retinal  area. 

The  centre  of  the  optic  papilla  not  infrequently  presents  a  funnel- 
shaped  depression,  at  the  bottom  of  which  the  retinal  vessels 
enter  ;  this  depression,  variable  in  size  and  form,  but  always  retain- 
ing sloping  walls,  is  known  as  the  physiological  excavation,  as 
distinguished  from  those  possessing  the  vertical  or  overhanging 
walls  indicative  of  grave  pathological  change. 

At  some  distance  (15-20  mm.)  from  the  eyeball  the  retinal  blood- 
vessels pierce  the  exterior  of  the  optic  nerve  to  take  up  a  central 


THE    EYE   AND    ITS   APPENDAGES. 


361 


position,  surrounded  by  connective  tissue,  which  they  maintain 
until  their  final  branching  on  the  papilla. 

The  blood-vessels  of  the  retina  constitute  an  independent  sys- 
tem composed  of  end-arteries  ;  the  only  communication  between 
the  retinal  and  ciliary  vessels  is  established  within  the  sclera,  close 
to  the  optic  nerve,  by  means  of  minute  scleral  and  choroidal 
branches.  The  larger  retinal  vessels  are  situated  within  the 
inner  part  of  the  fibre -layer  and  supply  twigs  to  the  cerebral 
division  alone,  the  epithelial  portion  being  non-vascular  and 
deriving  its  nutrition  from  the  adjacent  choriocapillaris. 

The  capillaries  are  arranged  as  two  net-works,  an  inner  and 
an  outer.  The  inner  net-work  lies  within  the  fibre-layer,  is  wide- 
meshed  and  derived  directly  from  the  division  of  the  retinal  vessels ; 
the  outer  net-work,  situated  within  the  inner  nuclear  layer,  is 
dependent  upon  the  former,  since  its  capillaries  are  derived  from  the 
branches  given  off  from  the  inner  vascular  reticulum.  The  retinal 
arteries  and  veins  are  surrounded  by  adventitious  sheaths,  the 
spaces  included  between  these  sheaths  and  the  walls  of  the  vessels 
constituting  perivascular  lymph-clefts. 

THE    CRYSTALLINE   LENS. 

The  crystalline  lens  comprises  two  genetically  distinct  portions, 
the  lens-substance  and  the  lens-capsule. 

The  lens-substance  consists  of  the  epithelium  of  the  lens  and 
the  lens-fibres — both  epithelial  structures  directly  derived  from  the 
invaginated  ectoderm. 

The  epithelium  of  the  lens,  the  representative  of  the  anterior 
wall  of  the  primary  lens-vesicle,  consists  of  a  single  layer  of  low 
polyhedral  cells,  about  20  p.  in  diameter,  whose  granular  proto- 
plasm contains  an  oval  nucleus,  also  often  vacuoles.  These  cells  lie 
immediately  beneath  the  anterior  capsule  and  extend  backward 
as  far  as  the  equator,  at  which  point  the  epithelial  cells  are  trans- 
formed into  the  lens-fibres.  A  thin  subcapsular  stratum  of 
albuminous  substance  exists  as  a  connecting  medium  between  the 
epithelium  and  the  capsule,  the  same  substance  being  continued  be- 
tween the  posterior  lens-capsule  and  the  lens-fibres  behind.  Be- 
neath the  epithelium  a  subepithelial  stratum  of  somewhat  simi- 
lar albuminous  substance  unites  the  epithelium  and  the  lens-fibres 
and  occupies  the  cleft  representing  the  remains  of  the  original 
cavity  of  the  lens-vesicle  ;  sometimes  a  few  drops  of  fluid — the 
liquor  Morgagni — occupy  this  subepithelial  stratum. 

The  lens-fibres  are  greatly  elongated  modified  epithelial  cells, 
whose  ancestors  constituted  the  posterior  wall  of  the  lens-sac,  but 
whose   more   recently  formed   fellows  result   from   the   transforma- 


-^2  NORMAL   HISTOLOGY. 

tion  of  the  peripherally  situated  anterior  epithelium  at  the  equator. 

They  are  elongated  compressed  six-sided  prisms  varying  in  size 

with  their  position  ;  those  at  the 
periphery  of  the  lens  are  the 
largest  (12  mm.  in  length  by 
10-12  fi  in  breadth),  their  size 
decreasing  towards  the  centre. 
In  the  young  lens  all  the  fibres 
contain  oval  nuclei,  but  in  the 
adult  organ  only  those  recently 
formed  lying  in  the  vicinity  of 

Fig.  384. 


Portions  of  human  crystalline  lens  :  A ,  section 
through  periphery  at  equator;  a,  anterior  capsule  ; 
b,  anterior  epithelium  converted  into  lens-fibres  (I) 
at  equator  (z)  ;  n,  nuclei  of  young  lens-fibres.  B, 
fragment  of  anterior  capsule  with  adherent  epi- 
thelium, viewed  from  under  surface  ;  h,  capsule  ;  e, 
epithelial  cells. 


Fibres  of  human  crystalline  lens  :  A  , 
portions  of  young  isolated  fibres  ;  B,  fibres 
in  transverse  section. 


the  equator  possess  these.  The  fibres  constituting  the  softer 
cortical  zone  have  smooth  straight  contours,  while  those  of 
the  central  part  display  a  finely-serrated  outline  and  are  with- 
out nuclei.  The  lens-fibres  are  united  by  albuminous  cement- 
substance,  which,  after  suitable  maceration,  is  dissolved,  so  that  the 
fibres  may  be  readily  isolated  ;  since  the  amount  of  the  cement-sub- 
stance is  less  between  the  broader  than  between  the  narrow  surfaces 
of  the  fibres,  after  suitable  maceration  the  lens  evinces  a  disposition 
to  separate  into  concentric  lamellae,  somewhat  after  the  fashion  of 
an  onion.  The  apposition  of  the  ends  of  the  fibres  takes  place  along 
definite  lines  which  appear  on  the  anterior  and  posterior  surfaces  of 
the  lens  as  stellate  figures,  the  lens-stars.  In  the  simpler  con- 
ditions of  the  new-born  child,  as  well  as  in  most  mammalia,  each  star 
consists  of  three  rays,  one  of  which  in  the  anterior  star  is  directed 
upward,  while  the  others  are  disposed  at  an  angle  of  1200  down  and 
outward  ;  in  the  posterior  star  the  rays  form  an  angle  of  6o°  with 
those  of  the  anterior  surface,  so  that  the  figures  of  both  surfaces 


THE   EYE  AND   ITS   APPENDAGES.  ,5^ 

together  constitute  a  six-rayed  star.  In  the  adult  lens,  however, 
the  typical  arrangement  of  the  rays  is  greatly  complicated  by  the 
addition  of  secondary  lines  which  obscure  the  figures. 

The  capsule  of  the  lens  is  a  strong  transparent  elastic  mem- 
brane completely  enclosing  the  lens  and,  at  the  periphery,  intimately 
uniting  with  the  suspensory  fibres  of  the  zone  of  Zinn.  The  an- 
terior capsule  covering  the  front  lens-surface  is  thicker  (11-15  p.) 
than  the  corresponding  posterior  capsule  (5-7  fi),  the  maximum 
thickness  being  at  the  centre  of  the  anterior  lens-surface  and  the 
minimum  at  its  posterior  pole. 

The  zone  of  Zinn,  zonula  ciliaris,  or  suspensory  ligament 
of  the  lens,  is  the  radially  plicated,  modified  anterior  continuation 
of  the  hyaloid  membrane  of  the  vitreous  body.  At  the  ora  ser- 
rata  the  hyaloid  becomes  intimately  united  to  the  posterior  surface 
of  the  ciliary  body  as  far  as  the  ciliary  processes,  from  whose  sum- 


Fig.  385 


Section  through  anterior  segment  of  human  eye,  including  cornea,  sc'era,  iris,  ciliary  body,  and 
lens:  a,  i,  substantia  propria  of  cornea  (C)  and  of  sclera  (S) ;  c,  sclero-corneal  juncture ;  d,  conjunc- 
tival tissue  ;  e,  stroma  of  iris  (/) ;  f,  connective  tissue  of  ciliary  processes  (g) ;  h,  canal  of  Schlemm  ; 
k,  trabecular  connecting  sclera  and  ciliary  body ;  /,  section  of  blood-vessel ;  m,  n,  o,  meridional, 
radial,  and  circular  fibres  of  ciliary  muscle ;  p,  continuation  of  hyaloid  membrane  into  ligament  (r) 
of  lens  (L)  ;  s,  spaces  of  Fontana ;  t,  muscular  tissue  of  pupillary  sphincter;  u,  pigment-layer  mark- 
ing termination  of  retinal  layers  at  pupil. 


mits  thickened  bands  bridge  across  the  intervening  space  and  become 
attached  principally  to  the  anterior  surface  and  to  the  periphery  of 
the  lens.     Owing  to  the  plication  of  the  ciliary  body  over  which  the 


,54  NORMAL    HISTOLOGY. 

hyaloid  is  reflected,  its  surface  is  marked  by  radiating  folds,  which 
at  the  edge  of  the  ciliary  processes  become  converted  into  the  stiff 
fibres  distinguishing  the  free  part  of  the  zonula.  These  fibres  form 
two  series,  the  one  comprising  the  fibres  springing  from  the  sum- 
mit of  the  ciliary  processes,  the  other  consisting  of  those  fibres 
which  take  their  origin  in  the  depressions  between  the  ciliary  pro- 
cesses ;  the  fibres  extending  from  the  valleys  pass  to  the  anterior 
surface  of  the  lens,  where  they  blend  with  the  outer  lamella  of  the 
anterior  capsule,  while  those  springing  from  the  summits  of  the 
processes  are  inserted  into  the  periphery  and  the  immediately  ad- 
joining parts  of  the  posterior  capsule. 

The  narrow  annular  cleft,  triangular  in  section,  bounded  in  front 
by  the  zone  of  Zinn,  mesially  by  the  lens,  and  behind  by  the  mem- 
brane of  the  vitreous  body,  constitutes  the  canal  of  Petit.  Owing 
to  the  constrictions  produced  by  the  shorter  bridging  fibres,  the  canal 
presents  a  series  of  alternate  constrictions  and  dilatations, 
which,  on  inflation,  map  out  the  position  of  the  canal  by  a  ring  of 
bead-like  enlargements. 

THE   VITREOUS   BODY. 

The  vitreous  body  occupies  the  space  between  the  lens  in  front 
and  the  retina  behind  ;  it  consists  of  the  vitreous  substance  en- 
closed by  the  glassy  hyaloid  membrane,  which  in  front,  where  it 
supports  the  lens  within  the  patellar  fossa  on  its  anterior  surface, 
comes  in  direct  contact  with  the  posterior  capsule. 

The  substance  of  the  vitreous  body  is  remarkable,  in  addition 
to  its  beautiful  transparency,  for  its  great  fluidity,  consisting  of  98.6 
per  cent,  of  water,  the  remaining  small  portion  being  made  up  of 
solids,  including  its  organized  parts.  Histologically,  the  adult  vitreous 
substance  corresponds  to  connective  tissue  containing  an  enormous 
watery  infiltration  whose  fixed  elements  have  undergone  degener- 
ation. In  its  embryonal  condition  the  vitreous  body  is  composed 
of  delicate  gelatinous  or  mucoid  mesodermic  tissue  containing 
numerous  frail  stellate  cells. 

The  formed  elements  of  the  vitreous  are  of  two  kinds,  fibres 
and  cells.  The  fibrous  elements  occur  in  the  superficial  part  of 
the  vitreous,  in  the  vicinity  of  the  ora  serrata,  as  fibrillar  of  extreme 
delicacy,  which  take  part  in  the  formation  of  the  zone  of  Zinn. 
Other  fibrous  structures  are  present  as  the  remains  of  the  minute 
blood-vessels  permeating  the  vitreous  in  its  embryonal  condition. 

The  cells  of  the  vitreous  body  belong  to  the  category  of  wan- 
dering corpuscles  or  leucocytes,  the  fixed  connective-tissue  cells 
being  wanting  in  the  matured  organ. 

In  the  central  part  of  the  vitreous  body,  the  central  or  hyaloid 


THE   EYE   AND    ITS    APPENDAGES.  ^r 

canal  extends  from  the  optic  papilla  to  the  vicinity  of  the  posterior 
lens-capsule  ;  during  fcetal  life  it  transmits  the  hyaloid  artery,  and 
afterwards  contains  the  remains  of  the  supporting  connective  tissue, 
and,  rarely,  the  atrophic  artery  itself.  The  canal  is  defined  by  a  thin 
membranous  wall,  the  continuation  of  the  hyaloid  membrane. 
The  existence  of  other  additional  small  lymphatic  spaces  has 
been  demonstrated  within  the  periphery  of  the  vitreous  body. 

The  minute  arrangement  and  ultimate  distribution  of  the  blood- 
vessels in  the  various  parts  of  the  eye  have  already  been  described 
in  connection  with  the  individual  structures  ;  it  here  remains  to  out- 
line briefly  the  general  relations  of  the  larger  trunks. 

The  blood-vessels  of  the  eyeball  belong  to  two  distinct  systems, 
the  retinal  and  the  ciliary,  which  are  connected  by  meagre  anasto- 
moses only  around  the  optic  nerve  entrance,  otherwise  they 
remain  entirely  separate. 

The  retinal  system  is  formed  by  the  ramifications  of  the  reti- 
nal artery  and  vein,  which  constitute  the  permanent  circulation 
within  the  nervous  layer.  During  fcetal  life  an  additional  transient 
supply,  represented  by  the  hyaloid  artery,  is  distributed  to  embryo- 
nal structures  which  disappear  before  birth. 

The  ciliary  system  consists  of  the  ramifications  of  the  short,  the 
long,  and  the  anterior  ciliary  arteries  and  their  complementary  veins, 
and  furnishes  the  blood-supply  to  the  bulbar  conjunctiva,  the  sclera, 
the  choroid,  the  ciliary  body,  and  the  iris,  and  indirectly  aids  in 
maintaining  the  nutrition  of  the  cornea,  the  lens,  and  the  epithelial 
division  of  the  retina. 

The  short  ciliary  arteries  supply  principally  the  choroid,  and 
form  the  choriocapillaris,  at  the  same  time  giving  off  twigs,  before 
piercing  the  sclerotic  coat,  to  the  posterior  segment  of  the  sclera  and 
to  the  dural  sheath  of  the  optic  nerve.  The  long  ciliary  arteries 
pierce  the  sclera  and  pass  in  the  horizontal  meridian  between  the 
scleral  and  choroid  coats  as  far  forward  as  the  ciliary  body,  in  which 
they  form  the  larger  arterial  circle  of  the  iris  ;  additional  recurrent 
twigs  are  given  off  to  the  choroid  and  the  ciliary  muscle.  The  larger 
arterial  circle  sends  branches  to  the  ciliary  processes  and  to  the  iris, 
as  well  as  a  few  twigs  to  the  choroid. 

The  anterior  ciliary  arteries  pass  to  the  anterior  segment  of  the 
ball,  and  pierce  the  sclera  near  the  corneal  margin  to  gain  access  to 
the  ciliary  muscle  behind  the  canal  of  Schlemm.  Before  entering 
the  eyeball  they  send  branches  to  the  anterior  segment  of  the  sclera, 
to  the  scleral  conjunctiva,  and  to  the  corneal  limbus.  From  the 
branches  which  pierce  the  eyeball  twigs  communicate  with  the  larger 
arterial  circle  of  the  iris,  and  supply  the  ciliary  muscle  and  the  fore 
part  of  the  choroid. 


366 


NORMAL   HISTOLOGY. 


The  venous  vessels  of  the  eyeball  culminate  in  two  principal 
sets,  the  posterior  and  anterior  ciliary  veins.  The  former,  or  the 
venae  vorticosae,  collect  the  blood  from  the  iris,  the  ciliary  processes, 
part  of  the  ciliary  muscle,  and  the  choroid,  and  on  emerging  from 
the  sclera  receive  also  the  episcleral  veins  ;  they,  therefore,  drain  the 
entire  territory  supplied  by  the  ciliary  arteries,  except  a  part  of  the 
region  nourished  by  the  anterior  ciliary  arteries. 

The  lymphatics  of  the  eyeball  constitute  the  anterior  and  pos- 
terior lymph-tracts,  which  do  not  comprise  definite  lymphatic 
vessels,  but  a  series  of  intercommunicating  lymph-spaces  varying  in 
size  from  the  microscopic  tissue-spaces  to  the  anterior  chamber. 

The  anterior  lymph-tract  includes  : 

i.  The  systems  of  the  lymph-spaces  within  the  cornea  and  the 
sclera. 

2.  The  anterior  chamber  of  the  eye,  containing  the  aqueous 
humor,  which  possesses  in  small  number  the  usual  histological  ele- 
ments of  lymphatic  fluid,  the  leucocytes.  The  anterior  chamber  com- 
municates with  the  posterior  chamber  through  the  cleft  between  the 
iris  and  the  lens,  and  indirectly,  by  means  of  the  spaces  of  Fontana, 
with  the  canal  of  Schlemm. 

3.  The  canal  of  Petit,  connected  by  means  of  the  interfascicular 
clefts  with  the  posterior  chamber,  and  thus  indirectly  with  the  ante- 
rior, these  three  spaces  standing  in  close  relation. 

The  posterior  lymph-tract  includes  two  groups,  the  lymphatics 
of  the  retina  and  of  the  vitreous  body  and  those  of  the  pericho- 
roidal space. 

The  constituents  of  the  first  group  are  : 

1.  The  hyaloid  canal  of  the  vitreous,  which  empties  into  the 
lymph-clefts  of  the  optic  nerve. 

2.  The  perivascular  lymph-channels  surrounding  the  retinal 
vessels,  which  likewise  pour  their  contents  into  the  lymph-spaces  of 
the  nerve. 

3.  The  lymph-clefts  of  the  optic  nerve,  terminating  within  the 
subarachnoidean  space  of  its  sheaths. 

The  perichoroidal  space,  lying  between  the  scleral  and  the 
choroid  coat,  drains  the  choroid  and  communicates  with  the  sac 
enclosed  by  Tenon's  capsule  ;  the  perivascular  lymphatics  sur- 
rounding the  venae  vorticosae  lead  from  the  perichoroidal  cleft  into 
Tenon's  space,  from  which  channels  connect  with  the  supra- vaginal 
space,  embracing  the  optic  nerve  ;  finally,  communications  exist 
between  this  space  and  the  great  intercranial  lymphatic  cavities. 
Connections  between  the  lymph-clefts  of  the  optic  nerve  and  the 
perichoroidal  space  probably  also  exist  in  the  vicinity  of  the  optic 
entrance. 


THE   EYE   AND   ITS   APPENDAGES.  -5- 

The  nervous  supply  of  the  several  parts  of  the  eye  has  already 
been  considered  in  detail ;  it  remains  to  add  a  short  description  of 
their  general  relations. 

The  long  and  short  ciliary  nerves  pierce  the  sclerotic  coat  in 
the  vicinity  of  the  optic  nerve  and  pass  between  the  sclera  and  the 
choroid,  giving  off  branches  for  the  supply  of  the  latter,  and  unite 
to  form  the  ciliary  ganglionic  plexus  on  the  outer  part  of  the 
ciliary  body.  From  this  plexus  twigs  pass  to  the  tissues  of  the  ciliary 
muscle,  the  iris,  and  the  cornea,  to  be  distributed  in  the  manner 
already  described. 

THE  APPENDAGES   OF   THE   EYE. 
THE    EYELIDS. 

The  eyelids  are  protecting  folds  which  include  between  their 
tegumental  and  mucous  surfaces  connective  tissue,  muscular  and 
glandular  structures.  The  constituents  of  the  eyelids  are  arranged 
as  general  layers  from  without  inward,  these  being  :  (1)  the  integu- 
ment and  subcutaneous  tissue,  (2)  the  muscular  layer,  (3)  the 
median  connective  tissue,  (4)  the  tarsal  plate,  and  (5)  the  con- 
junctiva. 

The  skin  covering  the  external  surface  of  the  eyelid  is  thin,  thrown 
into  folds,  and  beset  with  fine  hairs  and  small  sweat-glands  ;  the 
corium  possesses  slightly-developed  papillae,  except  at  the  edge  of 
the  lid,  where  the  fibrous  tissue  is  denser  and  displays  more  conspic- 
uous elevations.  The  constant  occurrence  of  pigment-cells  within 
the  corium  is  a  noteworthy  peculiarity. 

The  loose  subcutaneous  tissue  is  rich  in  elastic  fibres,  but  fat  is 
wanting,  or,  if  present,  is  found  only  in  meagre  amount.  At  the 
outer  border  of  the  margin  of  the  lid  large  stiff  hairs,  the  cilia,  ex- 
tend obliquely  outward  ;  they  are  arranged  as  two  or  three  rows, 
their  hair-follicles  extending  deeply  into  the  corium  and  being  sup- 
plied with  small  sebaceous  glands.  The  life  of  the  cilia  is  short, 
being  about  four  months  in  duration  ;  as  a  result,  hairs  in  all  stages 
of  growth  are  usually  included  among  the  eyelashes. 

The  muscular  bundles  of  the  orbicularis  palpebrarum  constitute 
the  layer  next  the  subcutaneous  tissue.  At  the  lower  margin  of  the 
lid  the  muscle-bundles  are  divided  by  the  outer  structures  occupying 
this  region  ;  an  especially  robust  bundle  separated  by  the  lashes  lies 
near  the  posterior  margin  of  the  lid-edge  and  constitutes  the  ciliary 
or  marginal  muscle  of  the  lid. 

The  succeeding  connective-tissue  layer  is  composed  largely  of 
the  fibrous  extensions  of  the  tendon  of  the  levator  palpebrse, 
which  are  partly  inserted   into   the   areolar   tissue — fascia  palpe- 


368  NORMAL   HISTOLOGY. 

bralis — and  partly  attached  to  the  upper  edge  of  the  tarsus ;  the 
tarsal  portion  contains  bundles  of  non-striped  muscle,  which  col- 
lectively form  the  lid-muscle  of  Muller. 

The  tarsus  consists  of  a  semilunar  plate  of  dense  fibrous  tissue 

Fig.  386. 


Section  of  human  eyelid  :  a,  a,  skin ;  b,  subcutaneous  tissue ;  c,  cilium  ;  d,  median 
connective  tissue;  e,  tarsal  plate  containing  Meibomian  glands  (h);f,  tunica  propria 
of  conjunctiva  covered  by  its  epithelium  (g)  ;  i,  duct  of  Meibomian  glands  ;  J,  Moll's 
glands ;  m,  m,  cut  fibres  of  orbicular  muscle  ;  m',  marginal  bundle  of  same;  n,  sections 
of  sweat-glands  ;  o,  hairs  ;  i,  anterior  boundary  of  tarsus. 


lying  immediately  in  front  of  the  conjunctiva,  and  extending  as  a 
firm  but  elastic  lamina  from  the  sharply-defined  palpebral  border 
deeply  into  the  substance  of  the  lid.  The  tarsus  is  composed  of 
closely-felted    bundles  of  dense   fibrous    tissue,  whose   tough 


THE    EYE   AND    ITS    APPENDAGES.  ^5g 

resistant  mass  gives  form  and  support  to  the  softer  tissues  of  the  lids 
and  partly  covers  the  Meibomian  glands  embedded  within  its  sub- 
stance. 

The  Meibomian  or  tarsal  glands  constitute  a  series  of  about 
thirty  elongated  tubulo-acinous  structures  embedded  within  the 
substance  of  the  tarsal  plate,  nearer  the  anterior  than  the  poste- 
rior surface.  Each  gland  consists  of  a  long  vertical  duct,  whose 
general  course  is  perpendicular  to  the  margin  of  the  lid  ;  into  this 
canal  numerous  short  lateral  tubular  acini  open.  Since  the  ex- 
tremities of  the  glands  occupy  the  outer  arched  border  of  the  tarsus, 
these  structures  are  longest  in  the  middle  of  the  lid  and  progressively 
shorten  towards  either  end.  The  ducts  open  on  the  straight  pal- 
pebral border  as  a  row  of  minute  orifices  situated  parallel  to, 
but  at  some  little  distance  from,  the  sharply-defined  inner  palpebral 
border.  In  their  histology  the  Meibomian  glands  so  closely  re- 
semble the  sebaceous  follicles  of  the  skin  that  they  must  be  re- 
garded as  modifications  of  these  structures  ;  their  secretion  consists 
of  a  fatty  substance  similar  to  the  sebum  lubricating  the  integu- 
ment. The  ducts  of  these  glands,  about .  i  mm.  in  diameter,  are  lined 
by  an  epithelium  possessing  the  character  of  the  surrounding  epi- 
dermis, while  the  acini  (.08-.  15  mm.)  contain  several  layers  of  poly- 
hedral cells,  most  of  which  are  in  various  stages  of  fatty  degen- 
eration. In  the  upper  part  of  the  tarsus,  especially  in  the  nasal 
half,  additional  branched  tubular  glands  lie  partially  surrounded 
by  the  fibrous  tissue  ;  these  structures  correspond  in  composition  to 
the  tear-glands,  and  are  known  as  the  accessory  lachrymal  glands. 

The  conjunctiva  constitutes  the  innermost  layer  and  surface  of 
the  lid,  being  continuous  at  the  base  of  the  lid  with  the  bulbar  con- 
junctiva and  at  its  palpebral  border  with  the  integument.  The  con- 
junctiva consists  of  the  epithelium  covering  the  free  surface  and 
the  connective-tissue  matrix,  or  tunica  propria.  The  epithe- 
lium covering  the  inner  surface  of  the  lid  is  stratified  columnar  ; 
at  the  margin  of  the  lid  the  columnar  epithelium  passes  over  into 
the  squamous  cells  of  the  epidermis.  The  surface  of  the  conjunctiva 
covering  the  tarsal  plates  is  smooth,  but  beyond  its  epithelium  forms 
irregular  pockets,  which  in  section  somewhat  resemble  glands. 

Numerous  lymphoid  cells  within  the  reticulated  tunica  propria, 
in  certain  localities,  strongly  suggest  the  presence  of  diffuse  aden- 
oid tissue ;  the  amount  of  such  lymphoid  tissue  is  subject  to 
much  individual  variation  ;  it  is,  however,  usually  best  marked  in  the 
retrotarsal  portions  of  the  conjunctiva.  Circumscribed  lymph- 
follicles  are  occasionally  observed,  although  these  structures  are  less 
constant  in  man  than  in  many  of  the  lower  animals — dog,  cat,  sheep, 
or  ox. 

24 


--0  NORMAL   HISTOLOGY. 

Additional  minute  lymphoid  nodules  and  mucous  glands  occur 
within  the  conjunctival  fornix.  The  ocular  conjunctiva  pre- 
sents no  marked  differences  until  near  the  corneal  margin,  where  the 
epithelium  loses  its  columnar  character  and  assumes  the  stratified 
squamous  type  in  its  reflection  over  the  cornea. 

The  edge  of  the  lid  presents  two  borders,  the  outer,  rounded  off 
and  tegumental  in  character,  and  the  inner,  distinguished  by  its 
sharply-defined  margin  and  dense  fibrous  structure.  In  addition  to 
the  orifices  of  the  Meibomian  glands,  the  palpebral  border  is  pene- 
trated by  the  ducts  of  the  glands  of  Moll,  structures  properly 
regarded  as  modified  sweat-glands. 

The  vertical  fold  of  conjunctiva  occupying  the  inner  canthus,  the 
plica  semilunaris,  represents  the  third  eyelid,  or  membrana 
nictitans,  of  the  lower  animals.  In  exceptional  cases  the  base  of 
the  fold  contains  a  minute  plate  of  hyaline  cartilage  ;  a  small  race- 
mose gland,  the  homologue  of  the  Harderian  gland,  is  also  some- 
times present  at  the  base  of  the  semilunar  fold. 

The  lachrymal  caruncle  within  the  inner  canthus  is  an  isolated 
and  modified  island  of  skin,  possessing  an  epithelium,  a  corium, 
and  subcutaneous  tissue  similar  to  the  adjacent  integument  ;  the 
epithelium,  however,  is  without  the  stratum  corneum.  The  caruncle 
contains  adipose  tissue,  fine  hairs  with  relatively  large  hair-follicles, 
and  modified  sweat-glands  closely  resembling  the  glands  of  Moll. 
A  small  amount  of  involuntary  muscle  usually  exists  in  the  car- 
uncle, and  sometimes  a  few  additional  fibres  of  striped  muscle. 

The  blood-vessels  of  the  eyelids  pass  from  the  outer  and  inner 
angles  towards  the  centre  of  the  lid,  forming  an  arch,  the  arcus 
tarseus,  along  the  edge  of  the  lid,  and  a  second  anastomosis,  the 
arcus  tarseus  externus,  at  the  upper  margin  of  the  tarsal  plate  ; 
from  these  arterial  bows  smaller  twigs  are  given  off,  which,  in  addition 
to  supplying  the  integument,  the  Meibomian  glands,  and  the  glands 
of  Moll,  form  the  conjunctival  capillary  net-work  ;  additional  branches 
pass  to  the  fornix  conjunctivae  and  to  the  conjunctiva  of  the  eyeball. 

The  lymphatics  of  the  lid  are  arranged  as  two  sets  :  the  close- 
meshed  conjunctival  net-work  within  the  tarsal  mucous  membrane, 
and  the  wide-meshed  peritarsal  net-work  on  the  front  of  the  tarsus 
at  its  upper  border.  The  first  set  include  the  lymphatics  running 
near  the  palpebral  border,  as  well  as  the  narrow  channels  surrounding 
the  Meibomian  glands.  The  conjunctival  lymph-vessels  communi- 
cate with  the  peritarsal  net-work  by  means  of  the  coarse  reticulum 
within  the  tarsus  surrounding  its  glands,  as  well  as  by  direct  connec- 
tions established  by  the  twigs  which  pierce  the  tarsus  to  join  the 
net-work  within  the  conjunctiva.  The  peritarsal  lymphatics  possess 
valves. 


THE    EYE   AND    ITS    APPENDAGES.  ,-rj 

The  nerves  of  the  eyelids  form  the  rich  marginal  plexus  close 
to  the  palpebral  border  ;  the  trunks  taking  part  in  the  formation  of 
this  plexus  before  their  union  give  off  branches  to  the  orbicular 
muscle  and  the  skin,  as  well  as  additional  twigs  for  the  supply  of  the 
conjunctiva.  From  the  plexus  itself  fibres  are  distributed  to  the 
hair-follicles  of  the  cilia,  the  Meibomian  glands,  the  tarsal  conjunc- 
tiva, and  the  tissues  of  the  edge  of  the  lid.  The  ultimate  nervous 
distribution  includes  the  formation  of  subepithelial  net-works  of  fine 
non-medullated  fibres,  together  with  the  special  endings,  the  spherical 
end-bulbs,  occurring  within  the  bulbar  conjunctiva. 

THE    LACHRYMAL    APPARATUS. 

The  lachrymal  apparatus  includes  the  lachrymal  gland  and  the 
system  of  canals  carrying  off  the  fluid  secreted  under  usual  con- 
ditions. 

The  lachrymal  gland  represents  the  serous  racemose  type, 
closely  resembling  the  true  salivary  glands  in  structure  ;  the  organ 
differs  from  the  usual  racemose  gland  in  the  independent  course 
and  the  number  of  its  ducts,  of  which  about  a  dozen  are  usually 
present.  It  appears,  therefore,  more  accurate  to  regard  the  lachry- 
mal gland  as  a  group  of  closely-placed  small  individual  racemose 
glands  rather  than  as  a  single  organ. 

The  ducts  of  the  lachrymal  gland  are  lined  by  simple  columnar 

epithelium .    The  structure  of  the  acini 

Fir  i^" 
and  the  relations  of  their  groups  corre- 
spond to  those  of  the  serous  salivary 
glands,  the  secreting  cells  possessing 
similar  spherical  forms  and  granular  pro- 
toplasm. 

The  blood-vessels  of  the  lachrymal 
gland  form  the  usual  capillary  net-works 
supplying  the  acini  and  their  secreting 
cells.  "' 

i  i  i        j  Section  of  human  lachrymal  gland: 

The  nerves  distributed  to  the  glandu-  a>  acini;  limited  by  basement-mem- 
lar  tissue  pass  between  the  acini  and  form     branes  («0  and  lined  by  secreting  ceils 

,  ,  ( g) ;  i,  interacinous  connective  tissue. 

net-works  beneath  the  basement-mem- 
brane ;  their  ultimate  relations  to  the  secreting  cells  are  uncertain. 

The  lachrymal  canals  or  canaliculi  consist  of  three  coats — the 
epithelium,  the  tunica  propria,  and  the  muscular  tissue.  The 
epithelium  is  stratified  squamous,  and  forms  a  layer  about  .12 
mm.  in  thickness,  in  which  the  deepest  cells  are  columnar  and  the 
superficial  greatly  flattened.  The  tunica  propria  is  composed  of 
bundles  of  fibrous  tissue  among  which  lie  especially  rich  circularly- 
disposed  elastic  net-works.     Outside  the  tunica  propria  the  lachry- 


*j2  NORMAL   HISTOLOGY. 

mal  canals  are  surrounded  by  a  layer  of  striped  muscle  derived 
from  that  part  of  the  orbicularis  known  as  Horner's  muscle  ;  this 
tissue  is  arranged  as  small  bundles,  which  possess  a  general  longitu- 
dinal course  parallel  with  the  axis  of  the  greater  part  of  the  lachry- 
mal canals.  The  vertical  papillary  division  of  the  tube,  however, 
lies  at  right  angles  to  the  muscle-bundles,  which,  consequently,  seem 
to  enclose  this  part  of  the  canal  within  circular  or  sphincter  fibres  ; 
some  of  these  occupy  the  edge  of  the  lid  and  surround  the  puncta 
with  muscular  loops. 

The  mucous  membrane  of  the  lachrymal  sac  and  of  the  naso- 
lachrymal  duct  is  connected  with  the  periosteum  of  the  neighbor- 
ing bony  surfaces  by  loose  areolar  tissue,  within  which  is  lodged  a 
rich  venous  plexus. 

The  mucous  membrane  of  the  lachrymal  sac  and  of  the  duct 
partakes  largely  of  the  nature  of  lymphoid  tissue,  consisting  of  a 
connective-tissue  reticulum  infiltrated  with  lymphoid  cells.  From 
the  tear-sac  to  the  nasal  termination  of  the  duct  the  lining  epithe- 
lium is  stratified  columnar  in  character,  with  the  occasional  pres- 
ence of  cilia  within  the  lower  part  of  the  tube. 

The  eyeball  is  separated  from  the  surrounding  structures  within 
the  orbit  by  the  intervention  of  a  fibro-elastic  membrane  or  fascia, 
the  capsule  of  Tenon,  covered  by  a  continuous  layer  of  endothe- 
lial plates  ;  the  enclosed  episcleral  space,  or  space  of  Tenon, 
communicates  with  the  perichoroidal  space  on  the  one  hand  and 
with  the  supra-vaginal  cleft  on  the  other.  In  effect,  the  capsule  of 
Tenon  corresponds  to  a  synovial  sac,  whose  lubricated  surfaces  of 
contact  facilitate  the  movements  of  the  eyeball. 

DEVELOPMENT    OF   THE    EYE. 

The  earliest  indication  of  the  visual  organ  is  the  optic  vesicle,  a 
large  diverticulum  extending  on  either  side  from  the  primary  anterior 
brain-vesicle,  and  later  becoming  connected  by  a  constricted  stalk 
with  the  interbrain,  or  thalamencephalon. 

In  the  early  stage  the  optic  vesicle  lies  in  contact  with  the  ectoderm 
reflected  over  the  prominently  protruding  optic  diverticulum,  the  sur- 
rounding mesoderm  at  first  showing  no  differentiation.  Shortly  after 
the  optic  vesicle  has  reached  the  surface  ectoderm  the  latter  exhibits 
proliferation  and  thickening  opposite  the  external  pole  of  the  vesicle. 
This  ectodermic  area,  the  earliest  trace  of  the  future  crystalline 
lens,  soon  becomes  depressed,  the  invagination  progressing  until 
the  pit-  and  the  cup-stage  give  place  to  the  closed  vesicle,  which 
finally  separates  from  the  ectoderm  and  lies  beneath  the  surface  as 
the  lens-sac. 

Simultaneously  with  the  progress  of  these  changes  in  the  ectoderm, 


THE    EYE   AND    ITS    APPENDAGES. 


373 


Section  through  head 
of  ten-day  rabbit  em- 
bryo, exhibiting  primary 
optic  vesicle  (0)  pro- 
truding from  fore-brain 
(B)  and  coming  in  con- 
tact with  surface  ecto 
derm  (e)  :  m,  surround- 
ing mesoderm. 


Section  through  develop- 
ing eye  of  eleven-day  rab- 
bit embryo  :  B,  fore-brain 
connected  by  stalk  with 
optic  vesicle  io),  whose 
anterior  wall  is  partly  in- 
vaginated ;  /,  thickened  and 
depressed  lens-area. 


the  anterior  segment  of  the  primary  optic  vesicle  undergoes  an 
important  invagination,  whereby  the  front  wall  of  the  sac  is  pushed 
into  the  cavity  of  the  vesicle 
until  eventually  the  anterior 
and  posterior  walls  are  in 
apposition  and  the  included 
cavity  is  largely  obliterated. 
The  new  space  within  the 
indented  anterior  walls  of  the 
sac  constitutes  the  second- 
ary optic  vesicle  and  corre- 
sponds to  the  later  vitreous 
chamber.  These  important 
changes  probably  are  not  en- 
tirely attributable  to  the  me- 
chanical influence  exerted  by 
the  developing  lens-sac  on  the 
closely-applied  optic  vesicle, 
but  must  be  referred  also  to 
deeply-lying  formative  forces. 

The  invagination  of  the  optic  vesicle  is  not  confined  to  the 
anterior  pole,  but  takes  place  likewise  along  the  under  side  of  the 
sac  as  well  as  along  the  optic  stalk  ;  in  consequence  the  vesicle  is 
imperfectly    closed   below,    the 

cleft,    or    choroidal    fissure,  Fig.  390. 

thus  established  affording  an 
entrance  for  the  surrounding 
mesodermic  tissue  which  takes 
part  in  the  production  of  the 
primary  vascular  structures  oc- 
cupying the  vitreous  chamber. 
The  relations  of  the  parts  to 
the  fissure  are  well  shown  in 
frontal  sections,  where  the  cleft 
appears  as  a  conspicuous  break 
in  the  continuity  of  the  walls  of 
the  vesicle. 

The  Retina.  The  layers  of 
the  optic  vesicle  very  soon  ex- 
hibit marked  difference  in  their 
rate  of  growth,  since  the  an- 
terior depressed  lamina  rapidly  overshadows  the  posterior  layer  by  its 
much  greater  thickness  and  more  active  proliferation.  The  posterior 
wall  becomes  reduced  in  thickness,  owing  to  the  increase  in  the  size 


Sagittal  section  through  developing  eye  of  eleven- 
and-a-half  day  rabbit  embryo,  exhibiting  choroidal 
fissure  (C)  through  which  mesodermic  tissue  (/«) 
reaches  interior  of  secondary  optic  cup :  o,  i,  outer 
and  inner  layers  of  optic  vesicle  ;  /,  lens-sac. 


^74  NORMAL    HISTOLOGY. 

of  the  sac,  and  later  is  distinguished  by  the  appearance  of  deeply- 
pigmented  granules,  which  mark  the  beginning  of  the  pigment- 
layer  of  the  retina,  to  the  formation  of  which  the  posterior  lamina 
of  the  optic  vesicle  is  entirely  devoted  ;  the  pigment  is  first  seen  in 
the  vicinity  of  the  lip  of  the  cup,  from  which  point  the  colored  par- 
ticles spread  towards  the  posterior  pole. 

The  invaginated  anterior  lamina  becomes  greatly  thickened  and 
differentiates  into  the  remaining  highly-specialized  layers  of  the 
retina.  The  process  by  which  these  are  formed  corresponds  in  the 
main  points  with  the  differentiation  of  the  nervous  centres,  the  re- 
sulting tissues  being  of  two  kinds,  the  supporting  neuroglia  and 
the  nervous  elements. 

The  retinal  lamina  early  presents  a  narrow  inner  zone,  dis- 
tinguished by  its  meagre  nuclei  as  contrasted  with  the  richly-nu- 
cleated broad  outer  division ;  this  latter,  next  the  pigmented 
lamina,  with  many  strata  of  nuclei,  differentiates  into  an  outer  layer 
characterized  by  small,  deeply-staining  nuclei,  and  an  inner  layer 
of  larger  elements.  The  outer  layer  subsequently  divides  into 
three  strata,  the  outer  nuclear,  the  outer  reticular,  and  the  inner 
nuclear,  while  the  inner  layer  produces  two  zones,  the  inner 
reticular  and  the  ganglion-cell. 

The  rods  and  cones  appear  later  as  minute  hemispherical  eleva- 
tions on  the  outer  surface  of  the  external  limiting  membrane,  and  at 
first  possess  their  inner  segments  alone,  the  outer  members  later 
growing  out  from  the  inner.  At  birth  in  many  animals  (as  cats, 
rabbits,  etc.)  the  rods  and  cones  are  wanting,  and  even  in  man  they 
are  rudimentary  ;  the  macula  at  birth  is  still  undifferentiated. 

The  nerve-fibres  of  the  retina  are  derived  probably  from  two 
sources,  from  the  neuroblasts  of  the  retina  itself  and  from  those  of 
the  interbrain.  The  hollow  optic  stalk  becomes  solid  and  con- 
verted into  the  primary  optic  nerve,  which  acquires  its  nerve-fibres 
from  the  ingrowing  and  outgrowing  processes  of  the  retinal  and  the 
cerebral  elements. 

The  retinal  blood-vessels  develop  within  mesodermic  tissue, 
which  spreads  over  the  inner  surface  of  the  nervous  layer  at  a  com- 
paratively late  period  ;  the  vessels  first  appear  around  the  optic 
nerve  and  spread  peripherally.  They  are  not  connected  primarily 
with  the  central  vessels  of  the  retina,  but  with  branches  entering  at 
the  periphery  of  the  nerve  (O.  Schultze). 

The  crystalline  lens  proceeds  from  the  ectodermal  vesicle 
already  noted.  The  walls  of  this  sac  very  early  exhibit  marked  va- 
riation in  thickness,  the  anterior  lamina  being  relatively  thin  and 
composed  of  a  single  layer  of  cuboidal  cells,  which  persist  as  the 
flattened  polyhedral  epithelium  of  the  anterior  lens-capsule. 


THE    EYE    AND    ITS    APPENDAGES.  *y 

The  posterior  wall  of   the  lens-sac  plays  the  active  role  in  the 
formation  of  the  lens-substance,   since  the  production  of  the  lens- 
fibres  is  entirely  due  to  the  transformation  of  its  greatly-elongated 
cells.       After    the    obliteration    of   the 
original    cavity   of   the   sac    has    been  Fig.  391. 

completely  effected   by  the   apposition  *,r  m  .  ,...-.* 

of  the  enormously-thickened  posterior  /f~"-: '_'._  '■'■_-}  :•.;/_'■'•■;  ''.■_■  >■  ■% 

wall  and  the  anterior  lamella,  the  lens        J&\?3^  '    -,-r^US^'^k  n 

further  increases  in  size  by  the  addition       £vt;  /  ;     ~    .  .yvT^ '\~~.;'-  ' 
of  new  fibres  at  the  equator,  where  the      T/Y'-':         -  •        :' ~-:-~^  " 
metamorphosis  of  the  epithelial  elements  "^-^-ilz^   "         .\Z~ '^ 

into  the  lens-fibres  is  continually  taking       .--T       .      ^    ""'    :: '/-/-//  ;  = 
place.  VJ'';.'^  '"'/•"•-"<V 

The  anterior  and  posterior  cap-  ^K^^--. '' •  -  -  ^^ ':'■''  r 
sules  of  the  lens  are   genetically  dis-  .'■-  -jJJ^^.'l 

tinct  from  the  lens-substance,  since  they  'x  ::^    , i/;'/ V:  3 

are  mesoblastic  in  origin  ;  for  a  time       Action  through  developing  e>e  of 

.  eleven-and-a-half-day    rabbit   embrvo  : 

they  are  closely  associated  with  the  tran-  Bi  fore-brain  connected  with  optic  vesi- 
sient   lamellae  of  vascular  mesodermic     cle  (a>  near!y  effaced  by  apposition  of 

....  ,  r  r     1  invaginated  anterior  segment  Oj  with 

tissue  which  invest  the  surfaces  of  the     posterior  wau  ^ .  h  iens-sac,  com- 

lens     and     Constitute     the     tUniCSe    VaS-       pletely  closed  and  separated  from  ecto- 

,  „,,.,.,  r       1  derm;  t,  tissue  within  secondarv  optic 

culosae.      The    development  of  the     OTp  derived  from  surroundiDg  meso_ 
fibrous  tunic — the  sclera  and  the  cor-     dermic;. 
nea — proceeds    from    the    surrounding 

mesoderm,  which  undergoes  condensation  immediately  around  the 
ectodermic  structures  representing  the  retina  and  the  lens.  The 
mesodermic  tissue  at  the  sides  of  the  anterior  segment  grows  be- 
tween the  epidermis  and  the  lens,  and  constitutes  a  layer  of  consid- 
erable thickness  ;  subsequently  this  sheet  becomes  unequally  divided 
by  the  appearance  of  a  cleft,  the  primary  anterior  chamber,  into 
two  laminae  of  unequal  thickness  ;  of  these  the  anterior  and  thicker 
becomes  the  cornea  and  the  posterior  and  thinner  the  connective 
tissue  of  the  iris  and  the  transient  vascular  tunic  of  the  lens. 

The  mesodermic  corneal  stratum  undergoes  specialization  into 
the  substantia  propria,  the  anterior  and  posterior  limiting  mem- 
branes, and  the  endothelium,  the  anterior  epithelium  alone  being 
ectodermic. 

The  choroid  and  the  iris  are  closely  associated  in  their  origin 
with  the  mesodermic  tract  producing  the  fibrous  tunic,  the  rich 
vascular  net-works  characterizing  the  choroid  appearing  relatively 
late.  The  iris  does  not  grow  forward  until  the  anterior  chamber 
begins  to  form,  when  it  proceeds  as  a  blunt  continuation  of  the 
choroidal  tract  ;  while  the  stroma  of  the  iris  is  contributed  by 
the  mesoderm,  the  pigment-layer  is  derived  from  the  extension 


~-£  NORMAL    HISTOLOGY. 

of  the  rudimentary  portions  of  the  optic  cup,  whose  double-layered 
lip  corresponds  in  position  with  the  pupillary  margin. 

The  vitreous  humor  is  derived  from  the  mesodermic  tissue  occu- 
pying the  interior  of  the  optic  cup. 
Fig.  392.  This  tissue  appears  very  early,  in 

consequence  of  the  ingrowth  of 
the  mesoderm  through  the  cho- 
roidal fissure  ;  the  early  vitreous 
possesses  delicate  branched  cells 
as  well  as  numerous  blood-vessels, 
and  corresponds  to  soft  embry- 
onal connective  tissue  ;  later  the 
corpuscles  and  blood-vessels  dis- 
appear and  the  mass  assumes  its 
characteristic  semi  -  fluid  almost 
structureless  condition.  The  pe- 
ripheral zone  of  the  vitreous  un- 
dergoes condensation  and  forms 
the  hyaloid  membrane,  which 
in  the  ciliary  region  becomes  thick- 
ened and  constitutes  the  suspen- 
sory ligament  of  the  lens,  or  the 
zone  of  Zinn. 

The  eyelids  develop  as  folds  of 
integument  above  and  below  the 
corneal  area  ;  these  grow  towards 
one  another  and  finally  fuse,  all 
epidermal  demarcation  for  a  time 
disappearing.  Shortly  b  e  f  o  r  e 
birth  the  centre  of  the  epithelial 
layer  undergoes  degeneration  and 
the  lids  become  permanently  separated. 

The  epithelium  of  both  the  tegumentary  and  conjunctival  sur- 
faces is  derived  from  the  ectoderm,  as  are  also  such  epidermal 
appendages  as  the  hairs  and  the  glands,  the  Meibomian  glands 
corresponding  to  sebaceous  follicles  in  their  formation. 


Section  through  developing  eye  of  thirteen- 
day  rabbit  embryo  :  e,  ectoderm ;  /,  lens,  con- 
sisting of  anterior  nucleated  division  repre- 
senting thin  front  wall  of  lens-sac  and  greatly 
thickened  posterior  division,  completely  filling 
cavity  of  sac  by  elongated  fibres  whose  nuclei 
present  crescentic  zone  («)  ;  /,  posterior  pig- 
mented layer;  r,  specialized  anterior  retinal 
layer;  i,  point  where  layers  of  optic  vesicle  be- 
come continuous  ;  n,  extreme  peripheral  section 
of  tissue  of  primitive  optic  nerve  connected  with 
vascular  tunic  (v)  occupying  posterior  surface  of 
lens;  m,  surrounding  mesoderm,  which  at  t 
grows  between  lens  and  retina. 


THE   ORGAN    OF    HEARING.  <,yj 


CHAPTER    XVIII. 


THE    ORGAN    OF    HEARING. 


The  complicated  organ  of  hearing  of  man  and  the  higher  animals, 
reduced  to  its  essential  factors,  consists  of  two  parts, — the  system  of 
intercommunicating  epithelial  tubes,  certain  parts  of  whose  walls  are 
differentiated  into  special  structures  for  the  perception  of  the  sound- 
waves, and  the  elaborate  conducting  apparatus  for  the  transmis- 
sion, direct  and  indirect,  of  the  sound-impulses  to  the  perceptive 
structures. 

THE   EXTERNAL   EAR. 

The  external  ear,  including  the  pinna  and  the  external  audi- 
tory canal,  possesses  a  bony  or  cartilaginous  basis  over  which 
extend  the  integument  and  a  layer  of  subcutaneous  tissue. 
The  cartilage  is  of  the  yellow,  elastic  variety,  forming  a  thin,  tough, 
yielding  plate,  displaying  the  various  depressions  and  elevations  seen 
on  the  outside  ;  the  lobule,  however,  contains  no  cartilage,  but  only 
tough  fibrous  tissue  and  fat. 

The  skin  covering  the  pinna  corresponds  with  the  surrounding 
integument  ;  within  the  auditory  canal,  however,  it  presents  some 
change.  The  skin  covering  the  cartilaginous  division  of  the 
meatus,  together  with  part  of  the  roof  of  the  bony  division,  is  char- 
acterized by  its  thickness,  the  subcutaneous  tissue  also  constituting 
a  layer  of  considerable  depth,  which  includes  some  fat  and  many 
bundles  of  dense  fibrous  tissue.  Fine  hairs,  with  relatively  very 
large  sebaceous  glands,  occur  in  all  parts  of  this  surface,  as  do  also 
the  ceruminous  glands,  which  constitute,  conspicuous  structures 
and  closely  correspond  to  the  glands  of  Moll  within  the  eyelid,  being, 
like  them,  modified  sweat-glands.  Their  long,  narrow  ducts  during 
early  life  open  with  the  sebaceous  glands  into  the  hair-follicles,  but 
later  acquire  independent  orifices.  The  ceruminous  glands  pos- 
sess a  well-marked  basement-membrane,  within  which  lies  a  single 
layer  of  cuboidal  epithelial  cells,  with  a  thin,  longitudinal  stratum 
of  non-striped  muscle-cells  interposed.  The  secreting  cells 
contain  numerous  brown  particles,  but  the  presence  of  fat  is  question- 
able, the  fatty  constituents  of  the  cerumen  being  probably  contributed 
by  the  adjoining  sebaceous  glands.  The  coiled  masses  of  the  gland- 
tubes  are  situated  within  the  subcutaneous  tissue,  where  they  some- 
times reach  as  far  as  the  cartilage  or  the  bone. 


.-3  NORMAL    HISTOLOGY. 

The  skin  covering  the  greater  part  of  the  bony  canal,   on   the 
contrary,    is  very   thin   and    intimately  united    to    the   periosteum. 

glands  are  want- 


Fig.  393. 


^^■^^-^fy&^f- '€■■ *-i 


sS>*Sv  <=»:" 


#3$; 


K<if: 


18SPS 


Hairs  and 

ing  in  this  part  of  the  canal,  as 
they  are  also  in  the  integument 
reflected  over  the  external  sur- 
face of  the  tympanic  membrane. 
The  membrana  tympani 
consists  of  three  layers  :  (1)  the 
central  ground-stratum,  or  lam- 
ina propria,  composed  of 
fibrous  connective  tissue,  (2) 
the  cutaneous  layer  reflected 
over  the  external  surface  of  the 
drum,  and  (3)  the  mucous 
layer  covering  the  inner  side 
of  the  membrane  as  the  repre- 
sentative of  the  lining  of  the 
tympanic  cavity. 

The  tegumental  layer  con- 
sists of  the  usual  epidermis 
and  connective-tissue  corium, 
the  latter  being  only  about  half 
as  thick  as  the  epithelial  layer. 

The  central  connective-tissue 
ground-plate,  or  lamina  pro- 
pria, constitutes  the  fibrous  basis 
of  the  tympanic  membrane  and 
represents  its  mesodermic  portion.  This  layer  consists  of  closely- 
felted  bundles  of  fibrous  tissue  arranged  as  two  strata,  the  outer  or 
radial  fibre-layer,  composed  of  fibrous  bundles,  which  in  their 
general  course  radiate  from  the  periphery  of  the  tympanum  towards 
the  point  of  attachment  of  the  head  of  the  malleus,  and  the  inner  or 
circular  fibre-layer,  consisting  of  concentrically-disposed  bundles, 
whose  greatest  development  is  at  the  periphery  in  the  vicinity  of  the 
annular  attachment  of  the  membrana  tympani. 

The  mucous  layer  is  a  part  of  the  general  lining  of  the  middle 
ear,  and  consists  of  a  thin  connective-tissue  tunica  propria  or 
groundwork,  composed  of  delicate  bundles  of  fibro-elastic  tissue, 
upon  which  rests  the  epithelium  ;  the  latter  consists  of  a  single 
layer  of  low  cuboidal  polyhedral  cells  without  cilia. 

The  blood-vessels  supplying  the  tympanic  membrane  are  derived 
from  two  sources,  the  one  set  proceeding  from  the  branches  of  the 
external  auditory  canal  to  end  in  capillaries  which  ramify  within  the 


■"**S 


0 


Section  of  bony  portion  of  human  external  audi- 
tory canal :  j,  cutaneous  layer  closely  united  with 
periosteal  fibrous  tissue  ;  o,  osseous  tissue  of  wall. 
(After  Riidinger.) 


THE    ORGAN    OF    HEARING. 


379 


cutaneous  layer,  the  other  group  coming  from  the  vessels  of  the 
tympanic  cavity  to  break  up  into  the  net-works  distributed  to  the 
mucous  layer. 

The  lymphatics  of  the  tympanum  correspond  in  their  arrange- 
ment with  the  principal  strata  of  the  membrane.  In  the  corium  of 
the  skin-layer  lies  a  close  net-work  of  capillary  lymphatics  ;  these 
increase  in  size  towards  the  periphery,  where  they  are  collected  into 
larger  trunks,  which  in  turn  empty  into  the  lymphatic  channels  of  the 

Fig.  394. 


Section  through  human  malleus  and  tympanic  membrane  :  1,  bony  tissue  of  manubrium,  containing 
medullary  canal  (2)  ;  3,  hyaline  cartilage  of  malleus ;  4,  5,  lamina  propria  of  tympanic  membrane 
attached  to  malleus  ;  6,  cutaneous  layer;  7,  mucous  membrane  covering  hammer;  8,  blood-vessel ; 
9,  fragment  of  fibro-cartilage.     (After  Riidinger.) 


external  auditory  canal.  Within  the  mucous  stratum  a  much  less 
important  lymphatic  net-work  exists,  which  communicates  at  the 
periphery  with  the  lymphatics  of  the  mucosa  of  the  tympanic  cavity. 
Suitable  silver  staining  shows  the  existence  of  lymph-spaces  in 
certain  places,  in  both  the  fibrous  layer  and  the  mucous  membrane. 

The  nerves  of  the  membrana  tympani  follow  the  blood-vessels  in 
their  distribution  so  far  that  they  also  comprise  two  sets  destined 
for  the  cutaneous  and  mucous  layers.  The  nerves  of  the  cutaneous 
stratum,  chiefly  derived  from  the  tympanic  branch  of  the  auriculo- 
temporal, pass  behind  the  manubrium  of  the  malleus  to  divide  at  the 


-3o  NORMAL    HISTOLOGY. 

lower  third  of  the  process  into  two  terminal  twigs.  In  addition  to 
these  central  nerves,  small  stems  enter  the  drum-membrane  at 
various  points  at  the  periphery,  both  sets  of  twigs  taking  part  in 
the  formation  of  a  wide-meshed  ground-plexus.  From  the  latter 
fine  pale  fibres  pass  to  the  blood-vessels  which  they  surround,  while 
other  fibres  extend  to  the  superficial  part  of  the  layer,  where,  be- 
neath the  epidermis,  they  constitute  a  subepithelial  plexus.  The 
nerves  of  the  mucous  layer  originating  in  the  tympanic  plexus  are 
largely  distributed  to  the  lymphatics  as  well  as  to  blood-vessels  ; 
an  additional  subepithelial  plexus  bears  close  relations  to  the  epi- 
thelium ;  a  few  fibres  extend  into  the  fibrous  tissue  of  the  lamina 
propria. 

THE    MIDDLE    EAR. 

The  middle  ear,  the  entodermic  division  of  the  auditory  appa- 
ratus, comprises  the  tympanic  cavity,  with  its  extension  into  the 
mastoid  cells,  and  the  Eustachian  tube,  together  with  the  series 
of  minute  ear-ossicles. 

The  walls  of  the  tympanic  cavity  consist  of  the  surrounding 
bony  structures  with  their  periosteum,  over  which  is  reflected  the 
mucous  lining,  indirectly  continuous  with  that  of  the  pharynx.  Th& 
mucous  membrane,  closely  united  with  the  underlying  periosteum, 
not  only  covers  the  inner  surface  of  the  membrana  tympani,  but  is 
also  reflected  over  the  ear-bones  and  their  ligaments  as  well  as  over 
the  nerves  and  blood-vessels  crossing  the  cavity.  The  mucosa  con- 
sists of  a  thin  fibrous  tunica  propria  (50-60  fi)  which  in  places 
resembles  the  reticulum  of  adenoid  tissue  and  includes  leucocytes  ; 
the  mucous  layer  is  intimately  blended  with  the  denser  fibrous  struct- 
ure of  the  periosteum.  Connected  with  the  trabeculae  of  the  mu- 
cosa peculiar  oval  bodies  are  occasionally  encountered,  which  are 
composed  of  an  axial  band  and  concentric  lamella?  of  connective 
tissue  ;  these  bodies  are  normal  but  probably  not  constant  constit- 
uents of  the  middle  ear. 

The  epithelial  lining  (18-21  ft)  of  the  tympanic  cavity  differs  in 
character  in  the  several  regions  ;  over  the  ear-ossicles,  the  tympanic 
membrane,  and  the  promontory,  as  well  as  within  the  mastoid  cells, 
the  epithelium  consists  of  a  single  layer  of  low  cuboidal  po- 
lygonal cells  without  cilia  ;  over  the  remaining  parts  of  the  mid- 
dle ear  a  layer  of  ciliated  columnar  cells  exists.  In  those  places 
where  nerve-trunks  or  blood-vessels  are  covered,  the  greatly-thick- 
ened mucosa  forms  local  ridges,  within  which  the  trunks  are  en- 
closed. Small  tubular  glands,  about .  1  mm.  in  length,  occur  in  the 
mucous  membrane  of  the  anterior  part  of  the  tympanic  cavity  ;  they 
are  sparingly  distributed  and  subject  to  individual  variation. 

The  mucous  lining  of  the  antrum  and  the  mastoid  cells,  clothed 


THE    ORGAN    OF    HEARING.  ^gl 

by  a  single  layer  of  low  polyhedral  cells,  is  very  thin  and  inti- 
mately united  with  the  delicate  periosteal  layer  ;  numerous  fibres, 
trabeculae,  or  lamellae  pass  between  neighboring  surfaces  and  partially 
occlude  the  spaces  within  the  bone,  thereby  reducing  the  lumina  and 
still  further  adding  to  the  complexity  of  the  mastoid  cells. 

The  secondary  tympanic  membrane,  closing  the  fenestra  ro- 
tunda, consists  of  three  layers,  a  central  fibrous  lamina  propria, 
which  is  covered  on  the  tympanic  surface  by  a  reflection  of  the  mu- 
cous membrane,  and  on  the  other  side  by  the  extension  of  the 
lining  of  the  vestibular  perilymphatic  space.  The  lamina  propria, 
the  unossified  part  of  the  wall  of  the  labyrinth,  is  composed  of  ra- 
dially-disposed bundles  of  fibrous  tissue  passing  from  the  indented 
point  of  its  base  towards  the  periphery.  The  mucous  stratum  is 
formed  of  a  thin  fibrous  tunica  propria  invested  by  a  single  layer 
of  flattened  non-ciliated  polyhedral  epithelial  cells,  similar  to 
those  covering  the  neighboring  promontorium.  The  innermost 
stratum  of  the  membrane  is  composed  of  a  thin  laver  of  sub- 
endothelial  fibrous  tissue,  over  which  extends  the  single  layer 
of  endothelial  plates. 

The  larger  blood-vessels  supplying  the  mucous  lining  of  the 
tympanum  lie  within  the  deeper  periosteal  layer  of  the  mucosa  and 
give  off  smaller  branches,  which  pass  superficially  to  form  a  capillary 
net- work  beneath  the  epithelium.  The  vessels  distributed  to  the 
mucosa  covering  the  promontorium  are  remarkable  for  the  absence 
of  anastomoses,  the  arteries  dividing  into  twigs  possessing  rela- 
tively large  lumina  ;  the  terminal  arterioles  pass  very  rapidly  into 
venous  radicles,  so  that  intervening  capillaries  scarcely  exist,  in  places 
being  entirely  wanting. 

The  lymphatics  of  the  tympanic  mucous  membrane  form  a  sys- 
tem of  channels  within  the  deep  periosteal  layer,  where  the  lymph- 
vessels  are  supplemented  by  spherical  enlargements  and  lateral  dila- 
tations. The  reticular  connective  tissue  of  the  mucosa  exhibits  local 
accumulations  of  lymphoid  cells,  which  strongly  suggest  the 
presence  of  lymphatic  nodules. 

The  principal  nerves  of  the  tympanum,  derived  from  the  tym- 
panic plexus,  run  within  the  periosteal  layer  of  the  mucosa,  and  are 
composed  almost  entirely  of  medullated  fibres.  From  the  deeper 
trunks  fine  twigs  pass  towards  the  surface  and  form  a  wide-meshed 
plexus,  which  contributes  delicate  bundles  of  pale  non-medullated 
fibres  to  a  subepithelial  net-work.  Along  the  course  of  the 
larger  trunks  and  their  immediate  branches  groups  of  ganglion-cells 
occur  in  exceptional  cases,  these  being  found  in  proximity  with  the 
epithelium. 

The  ear-ossicles  consist  of  compact  bone,  in  which  Haversian 


Fig.  395. 


0  NORMAL    HISTOLOGY. 

canals  and  concentric  lamellae  are  present  in  the  thicker  parts,  as  the 
head  and  the  base  of  the  short  process  of  the  malleus.  All  surfaces 
of  contact,  including  the  articular  facets,  are  invested  by  hyaline 
cartilage.  The  cavity  of  the  ambo-malleal  articulation  is  sub- 
divided by  a  minute  intra-articular  plate  of  fibrous  cartilage.  An 
investment  of  cartilage  covers  the  malleus  on  all  parts  of  the  sur- 
face of  its  attachment  to  the  tympanic  membrane,  the  perichondrium 
becomino-  firmly  united  with  the  fibrous  tissue  of  the  lamina  propria. 
The  entire  base  of  the  stapes  also  is  covered  with  a  plate  of  car- 
tilage directly  applied  to  the  fenestra  ovalis ;  the  space  intervening 
between  the  stapes  and  the  margin  of  the  oval  window  is  occupied 
by  the  ring  of  fibrous  tissue  constituting  the  annular  ligament. 

The  Eustachian  tube  consists  of  two  parts, — the  supporting 
framework,  composed  partly  of  bone  and  partly  of  cartilage,  and 

the  mucous  membrane.  Neither 
the  osseous  nor  the  cartilaginous 
tissue  of  the  canal  constitutes  a  com- 
plete wall,  since  the  tube  is  imperfect, 
being  completed  by  the  fibrous  and 
other  tissue  which  bridges  the  cleft  left 
by  the  insufficient  hard  parts. 

Within  the  canal  formed  by  the  os- 
seous, cartilaginous,  and  fibrous  tissues 
the  soft  tube  of  mucous  membrane 
lies,  its  lower  division  supplemented 
by  a  stratum  of  submucous  tissue, 
its  upper  part  closely  united  with  the 
periosteum  of  the  bony  walls. 

The  epithelium  lining  the  Eu- 
stachian tube  is  ciliated  stratified 
columnar  in  type,  the  cells  clothing 
the  pharyngeal  division  of  the  tube 
being  tall  columnar  elements,  while 
those  lining  the  upper  bony  part  are 
low  cuboidal,  although  ciliated,  and 
resemble  the  epithelium  of  the  tym- 


Section  through  cartilaginous  portion  of 
human  Eustachian  tube  :  1,  bent  plate  of 
cartilage  with  its  hook  (1')  ;  2,  fibrous 
tissue  with  fat  (3) ;  4,  tubo-pharyngeal 
fascia  ;  5,  dilator  tubae  muscle  ;  6,  mucous 
membrane  of  tube  with  prominent  fold 
(6')  below  ;  7,  mucous  glands  ;  8,  lumen 
of  tube  expanding  above  into  so-called 
safety-tube  (8')  ;  9,  connective  tissue 
uniting  tube  with  base  of  skull ;  10,  le- 
vator palati  muscle.     (Af:er  7  cstut.) 


panum. 
The 


tunica  propria  presents  a 
stratum  of  loose  connective  tissue, 
rich  in  cells  and  defined  from  the  sub- 
mucous tissue  by  a  denser  layer ;  in 
many  places  the  reticular  connec- 
tive tissue  is  infiltrated  with  lymphoid  cells  and  constitutes  an 
adenoid  structure.     The  profusion  and  distribution  of  this  lymphoid 


THE   ORGAN   OF   HEARING.  383 

tissue  vary  greatly  with  age  ;  in  early  childhood  it  is  present  almost 
in  all  parts  of  the  tube,  but  in  adolescence  it  is  plentiful  only  in  the 
lower  third,  in  the  upper  third  being  entirely  wanting  and  in  the 
middle  third  very  sparingly  distributed.  Small  raucous  glands 
are  also  present,  and  open  on  the  surface  of  the  mucosa  within  the 
depressions  between  the  longitudinal  folds  ;  these  glands  may  exist 
throughout  the  length  of  the  tube,  but  they  occur  with  constancy 
only  towards  its  pharyngeal  end. 

The  submucous  layer  is  well  developed  in  the  cartilaginous 
division  of  the  tube,  particularly  in  the  outer  membranous  wall  ;  it 
consists  of  loosely-arranged  fibro-elastic  tissue,  which  supports  the 
mucous  glands  and  the  larger  vessels  and  nerves,  and  often  contains 
a  considerable  mass  of  fat. 

The  blood-vessels  supplying  the  tubal  mucous  membranes  are 
derived  from  the  pharynx  and  from  the  tympanum  ;  the  larger  longi- 
tudinal stems  run  within  the  submucosa  or  the  deep  periosteal  layers 
and  send  twigs  into  the  mucosa  to  form  capillary  net-works. 

The  nerves  derived  from  the  pharyngeal  and  tympanic  plexuses 
occupy  the  deeper  layers  of  the  mucosa  ;  the  twigs  given  off  from 
the  larger  trunks  form  a  plexus  within  the  superficial  parts  of  the 
tunica  propria,  fine  non-medullated  fibrillar  passing  to  the  epithelial 
structures  ;  ganglion  nerve-cells  are  found  at  the  nodal  points  within 
the  plexus. 

THE    INTERNAL    EAR. 

The  internal  ear  in  its  fully-developed  condition  consists  of  two 
concentrically  arranged  parts,  the  bony  and  the  membranous 
labyrinth,  separated  by  an  intervening  space  containing  the  peri- 
lymph. 

THE   SACCULE   AND    THE   UTRICLE. 

While  the  bony  labyrinth  in  the  cochlea  and  the  semicircular 
canals  quite  closely  repeats  the  general  arrangement  of  the  corre- 
sponding parts  of  the  enclosed  membranous  tube,  the  central  divis- 
ion of  the  osseous  capsule,  the  bony  vestibule,  differs  somewhat 
in  its  details  from  the  enclosed  membranous  compartments. 

These  are  two  almost  completely  separated  vesicles  of  un- 
equal size,  the  anterior  and  smaller  sacculus  and  the  posterior  and 
larger  utriculus  ;  the  compartments  communicate  indirectly  with 
each  other  by  means  of  the  ductus  endolymphaticus,  while  the 
saccule  connects  additionally  with  the  cochlear  division  of  the 
membranous  labyrinth  through  the  narrow  canalis  reuniens,  the 
utricle  directly  opening  into  the  semicircular  canals. 

The  bony  wall  of  the  vestibule  is  lined  by  a  very  thin  perios- 
teum, composed  of  a  felt-work  of  resistant  fibrous  tissue,  in  which 
pigmented   connective-tissue  cells  are  not  infrequent.       From 


~g.  NORMAL    HISTOLOGY. 

this  peripheral  lamella  trabeculae  extend  across  the  intervening 
perilymphatic  space  to  the  fibrous  wall  of  the  membranous  laby- 
rinth. The  endothelium  of  the  inner  surface  of  the  periosteum 
invests  the  fibrous  trabeculae  as  well  as  the  outer  or  perilymphatic 
surface  of  the  membranous  labyrinth. 

The  walls  of  the  saccule  and  the  utricle  consist  of  the  con- 
nective-tissue lamella,  composed  of  the  bundles  of  fibrous  tissue 
and  the  delicate  epithelium.  At  the  positions  where  the  filaments 
of  the  auditory  nerve  enter  the  maculae  cribrosae  and  acusticee 
the  fibrous  stratum  is  best  developed  and  densest,  forming  a  layer 
.  15  to  .20  mm.  thick.  Within  other  parts  of  the  vestibule,  especially 
in  the  roof  of  the  utricle,  the  thickness  of  this  layer  may  be  reduced 
to  5-6  p.. 

The  lining  of  the  saccule  and  the  utricle  consists  everywhere, 
except  at  the  maculae  acusticae,  of  a  single  layer  of  thin  flattened 
polyhedral  cells.     Over  the  regions  receiving  the  terminations  of 


Fig.  396. 


v  '  • 


Section  through  membranous  labyrinth  of  cat,  showing  specialized  areas  within 
ampulla  {A)  and  utricle  (B)  :  a,  surrounding  bony  wall  separated  from  membranous 
tube  (6)  by  layer  of  areolar  trabecular  (d) ;  c,  crista  acustica  covered  with  specialized 
epithelium  (/);  e,  e' ,  bundles  of  nerve-fibres  ;  g,  ordinary  epithelium  ;  h,  layer  of 
otoliths  overlying  neuro-epithelium  of  macula  acustica  (/) ;  i,  blood-vessel ;  k,  fibrous 
layer  ;  /,  adipose  tissue. 

the  nerve-fibres,  the  maculae  acusticae,  on  the  contrary,  the  epi- 
thelium undergoes  marked  alteration,  changing  from  the  indifferent 
covering  cells  into  the  highly-specialized  neuro-epithelium.  At 
the  margin  of  these  areas  the  cells  are  at  first  cuboidal,  then  low 


THE   ORGAN   OF   HEARING.  385 

columnar,  and  progressively  increase  in  length  until  they  measure 
30-35  [i  in  contrast  with  their  usual  height  of  3-4  /-/.. 

The  character  and  arrangement  of  the  cells  of  the  acoustic  areas 
in  the  saccule  and  the  utricle  are  the  same,  including  two  kinds  of 
elements,  the  sustentacular  or  fibre  cells  and  the  hair-cells. 

The  sustentacular  cells  are  elongated  irregularly  cylindrical, 
and  extend  the  entire  thickness  of  the  epithelial  layer  to  rest  upon 
the  well-developed  basement-membrane  by  their  expanded  or  divided 
basal  processes.  The  oval  nuclei  are  frequently  wider  than  the 
average  diameter  of  the  cells,  and  produce  corresponding  enlarge- 
ments in  the  contour  of  the  elements  ;  the  nuclei  occupy  various 
levels  within  the  inner  half  of  the  cells,  but  are  never  situated  beyond  ; 
the  cell-contents  appear  faintly  granular,  and  contain  yellowish 
pigment-particles. 

The  hair-cells  are  broader  but  shorter  than  the  sustentacular  ele- 
ments, and  reach  from  the  surface  only  to  about  the  middle  of  the 
epithelium,  where  they  terminate  in  rounded  margins  ;  these  cells 
possess  large  spherical  nuclei,  which  usually  lie  well  towards  the 
slightly-expanded  inner  ends.  The  protoplasm  of  the  hair-cells  is 
granular,  and  contains  yellow  pigment ;  the  outer  part,  next  the 
free  surface,  exhibits  a  differentiation  into  a  cuticular  zone,  cov- 
ering the  outer  ends  of  the  cells.  From  the  free  border  of  each  cell  a 
seemingly  single  stiff  robust  hair  (20—25  p.  long)  projects  into  the 
endolymph  ;  this  conical  process,  how- 
ever, is  resolvable  into  a  number 
of  agglutinated  finer  hairs  or  rods. 

The  free  surface  of  the  neuro- 
epithelium  within  the  saccule  and 
the  utricle  is  covered  by  a  remark- 
able structure,  the  so-called  otolith 
membrane.  This  consists  of  num- 
berless small  crystalline  bodies,  the 
otoliths,  or  ear-stones,  embedded 
within  a  soft  gelatinous  ground-sub- 
stance.      The    otoliths    are    minute       Section  of  wal1  of  utricle  throush  mac" 

.  ular  region,  from  rabbit,  showing  otoliths 

Crystals    Of    Calcium    Carbonate,    I-I5   fl       (0)  embedded  within  granular  substance 

in    length,    usually    six-sided    prisms     W :  k>  hair-ceIls  with  processes  (/)  ex- 

~,  J  x  tending  between    sustentacular   elements 

With      sllghtly-rOUnded      angles.  The       (j)  ■  n,  nerve-fibres  within  fibrous  tissue 

nerve-fibres  proceed  to  the  acoustic     W  posing  towards  hair-ceiis  and  be- 

.  .  1     1.    1       coming    non-medullated     at     basement- 

areas  and  unite  beneath  the  epithelial     membrane  (;«)■ 
layer   in   a  plexus,   from   which  fine 

bundles  of  fibres  pass  towards  the  surface  ;  the  nerve-fibres  usually 
lose  their  medullary  substance  in  their  transit  through  the  base- 
ment-membrane and  enter  the  epithelium  as  naked  axis-cylinders. 

25 


,56  NORMAL   HISTOLOGY. 

After  ascending  about  half-way  to  the  free  surface  the  fibres  break  up 
into  their  fibrillae,  many  of  which  are  distributed  to  the  hair-cells, 
with  which  they  probably  stand  in  close  relation,  while  others  pass 
as  free  axis-cylinders  between  the  epithelial  elements  to  a  higher 
level. 

The  blood-vessels  of  the  saccule  and  the  utricle  form  a  wide- 
meshed  capillary  net-work  within  the  fibrous  wall  of  the  membranous 
sacs,  the  vascular  supply  being  especially  rich  within  the  maculae 
acusticae. 


THE   SEMICIRCULAR    CANALS. 

The  inner  surface  of  the  bony  capsule  constituting  this  part  of 
the  osseous  labyrinth  is  lined  by  a  thin  periosteum  similar  to  that 
of  the  vestibule  already  described.     Along  the  line  of  attachment  of 

the  membranous  canal  this  layer  sends 
off  numerous  connecting  bundles  of 
fibrous  tissue  ;  in  other  parts  of  the 
circumference  of  the  canal  only  widely- 
separated,  occasional  trabeculae 
bridge  across  the  perilymphatic  space 
to  aid  in  maintaining  the  position  of 
the  membranous  tube.  The  inner 
surface  of  the  periosteum,  the  tra- 
beculae, and  the  outer  face  of  the 
fibrous  tunic  of  the  membranous 
canals  are  invested  by  the  endothe- 
lium which  forms  the  immediate 
lining  of  the  perilymphatic  space. 
The  walls  of  the  membranous 
semicircular  canals  closely  re- 
semble those  of  the  saccule  and  the 
utricle,  being  made  up  of  an  outer 
fibrous  lamella  and  an  inner  epi- 
thelial lining.  The  fibrous  coat 
is  further  differentiated  into  an  external  layer  of  felted  connective- 
tissue  bundles,  containing  many  cells,  and  an  inner,  more  compact, 
almost  homogeneous  layer,  which  corresponds  to  a  highly-developed 
basement-membrane. 

The  epithelium  of  the  semicircular  canals,  supported  by  the  outer 
fibrous  coat,  consists  throughout  the  greater  part  of  its  extent  of  a 
single  layer  of  flattened  polyhedral  cells  (12-18  //.)  similar  to 
those  lining  the  saccule  and  the  utricle. 

The  areas  receiving  the  terminal  filaments  of  the  auditory  nerve, 
the  cristae  acusticae,  are  distinguished  by  specialization  of  the 


Section  of  wall  of  cat's  semicircular 
canal:  a,  epithelial  lining  of  canal;  b, 
basement-membrane ;  c,  fibrous  tunic 
united  with  osseous  lamella  (/)  by  tra- 
beculae (d,  d) ;  e,  blood-vessel. 


THE    ORGAN    OF    HEARING.  ,g7 

epithelium  to  constitute  the  perceptive  apparatus  of  the  semicir- 
cular canals. 

These  specialized  areas  are  limited  to  the  floor  of  the  ampullae, 
in  which  position  the  fibrous  wall  of  the  canal  is 
distinguished  by  a  local  thickening  forming  the 
transverse  ridge,  or  septum  trans versum. 

On  approaching  the  base  of  the  crest  the  epi- 
thelial cells  become  more  columnar,  being  much 
taller  and  narrower  than  those  of  the  general  sur- 
face. The  specialized  cells  crowning  the  sum- 
mits of  the  cristae  acusticae,  like  other  examples 
of  neuro-epithelium,  consist  of  elements  of  two 
kinds,  the  sustentacular  or   fibre  cells   and 

the  hair-Cells.  Surface  view  of  mem- 

rj^t  ,  1  ,  11,1  branous     semicircular 

The  sustentacular  elements  resemble  those    canal  of  cal .  x>  fibrous 
of  the  maculae  of  the  saccule   and   the   utricle,     tissue  supporting  single 
extending  the  entire  thickness  of  the   epithelial     ^L\cJis(y).  * 
layer     and     presenting    an    elongated     narrow 
irregular  cylindrical  body,  with  prominent  projecting  oval  nucleus. 

The  hair-cells,  or  auditory  cells,  reach  only  part-way  to  the 
basement-membrane  and  bear  on  their  free  surfaces  enormously  long 
hair-processes,  the  auditory  hairs,  which  project  at  least  as  far 
as  the  middle  of  the  lumen  of  the  ampulla.  The  auditory  hairs 
spring  from  the  ends  of  the  cells  by  minute  conical  expansions,  and 
under  high  amplification  are  resolvable  into  a  number  of  finer  com- 
ponent hairs.  The  nuclei  of  the  auditory  cells  usually  lie  within 
dilated  rounded  inner  extremities  of  the  cells,  with  which  also  the 
terminations  of  the  auditory  nerve  come  in  close  relations. 

In  sections  of  the  ampullae  in  tissue  preserved  with  approved 
reagents  the  inner  free  ends  of  the  auditory  hairs  are  embedded 
within  a  peculiar  dome-like  structure,  the  cupola,  regarding  whose 
nature,  and  even  existence  during  life,  opinions  greatly  differ.  As 
usually  seen  in  well-preserved  tissue,  this  structure  appears  as  a 
faintly-striated  cuticular  formation  covering  in  the  ends  of  the 
hair-processes. 

The  fibres  of  the  auditory  nerve  pass  into  the  septum  trans- 
versum,  where  they  unite  into  net-works  from  which  finer  diverging 
fibrillae  pass  into  the  overlying  epithelium  after  losing  their  medul- 
lary substance.  Small  groups  of  naked  axis-cylinders  extend  be- 
tween the  epithelial  cells  and  separate  into  the  individual  fibrillae, 
some  of  which  are  applied  to  the  bases  of  the  hair-cells,  while  others 
apparently  seek  their  ultimate  distribution  at  higher  levels. 

The  blood-vessels  supplying  the  semicircular  canals  comprise 
those  destined  for  the  bony  capsule  and  those  distributed  to  the 


,38  NORMAL   HISTOLOGY. 

membranous  structures.  These  vessels  form  a  wide-meshed 
capillary  net-work  within  the  fibrous  tunic  of  the  canals  and  the 
ampullae,  which  supplies  both  the  endo-  and  perilymphatic  surfaces. 

THE    COCHLEA. 

The  cochlea  consists  of  the  tapering  bony  tube  wound  spirally 
about  its  axis  and  containing  the  highly-specialized  but  much 
smaller  epithelial  canal,  the  ductus  cochlearis.    This  latter  tube, 

•»  F;g.  400. 


Longitudinal  section  of  cochlea  of  guinea-pig :  a,  bony  capsule ;  6,  central  shaft 
or  modiolus  ;  c,  lamina  spiralis  ;  d,  canal  of  modiolus  containing  bundles  of  nerve- 
fibres  (e) ;  f,  terminal  bundles  ;  g,  basilar  membrane  ;  h,  spiral  ligament  ;  /',  limbus  ; 
j,  membrane  of  Reissner  ;  I,  Corti's  organ  ;  m,  spiral  ganglion  ;  n,  blood-vessels  ;  V, 
T,  M,  respectively  scala  vestibuli,  tympani,  and  media. 

triangular  in  transverse  section,  is  attached  along  its  base  to  the  outer 
wall  of  the  bony  tube,  and  along  its  narrow  opposite  border  to  the 
projecting  osseous  spiral  lamina ;  in  consequence  of  this  arrange- 


THE   ORGAN    OF    HEARING. 


389 


ment  the  perilymphatic  space,  instead  of  constituting  a  single 
cavity  in  which  the  epithelial  tube  is  suspended,  is  divided  into  the 
scala  vestibuli  above  and  the  scala  tympani  below,  which  com- 
municate respectively  with  the  vestibule  and  the  tympanum. 

The  ductus  cochlearis,  or  scala  media,  consists,  like  other  por- 
tions of  the  membranous  labyrinth,  of  the  epithelial  tube,  the 
oldest  part  of  the  cochlea  representing  the  specialized  outgrowth 
from  the  primary  ectodermic  otic  vesicle,  and  the  supporting  fibrous 
tunic  derived  from  the  differentiated  surrounding  mesoderm. 

The  ductus  cochlearis  is  triangular  in  section ;  its  base  or  ex- 
ternal wall  is  attached  to  the  outer  wall  of  the  bony  capsule,  its 
apical  border  is  joined  to  the  end  of  the  osseous  spiral  lamina, 
and  the  converging  sides  are  formed  by  the  delicate  membrane  of 
Reissner  above  and  the  basilar  membrane  below,  which  separate 
respectively  the  scala  vestibuli  and  the  scala  tympani  from  the  scala 
media. 

The  vestibular  wall  of  the  cochlear  duct  is  formed  by  Reiss- 
ner's  membrane,  an  extremely  fragile  partition  dividing  the  duct 
from  the  scala  vestibuli  ;  the  membrane  begins  on  the  vestibular 
border  of  the  lamina  spiralis,  about  .2  mm.  inside  the  free  edge  of 
the  crista,  and  extends  at  an  angle  of  about  forty-five  degrees  until 
it  meets  the  outer  bony  wall.  Reissner' s  membrane  consists  of 
three  layers :  an  extremely  thin  central,  almost  homogeneous 
connective-tissue  stratum,  one  side  of  which  is  covered  by  the 
endothelium  of  the  vestibular  surface  and  the  other  by  the  epi- 
thelium of  the  cochlear  duct.  Notwithstanding  the  extreme  thin- 
ness of  this  layer,  the  presence  within  it  of  sparingly  distributed 
capillary  blood-vessels  has  been  demonstrated.  The  vestibular 
endothelium  consists  of  a  single  layer  of  delicate  plates,  which  here 
and  there  enclose  pigment.  The  surface  towards  the  duct  is  cov- 
ered by  the  general  ectodermic  lining  of  the  canal,  represented  by 
a  single  layer  of  flat  polyhedral  epithelial  cells.  The  three 
layers  contribute  equally  to  the  3  fi  representing  the  entire  thickness 
of  the  membrane. 

The  outer  wall  of  the  cochlear  duct  rests  against  a  greatly  thick- 
ened crescentic  cushion  of  connective  tissue,  whose  convex 
border  is  intimately  united  with  the  bony  wall,  and  whose  generally 
concave  margin  looks  towards  the  cochlear  duct.  This  area  of  con- 
nective tissue,  the  ligamentum  spirale,  extends  both  above  and 
below  the  boundary  of  the  cochlear  duct,  its  two  horns  forming  part 
of  the  outer  walls  of  the  adjacent  vestibular  and  tympanic  canals. 

The  concave  surface  of  the  ligamentum  spirale  is  interrupted 
opposite  the  level  of  the  tympanic  wall  of  the  cochlear  duct  by  a  pro- 
jecting ridge,  the  crista  basilaris  (Schwalbe),  to  which  the  basilar 


390 


NORMAL    HISTOLOGY. 


membrane  or  tympanic  wall  of  the  duct  is  attached.  Near  the  base 
of  the  basilar  crest  the  outer  wall  of  the  cochlear  duct  is  marked  by 
an  additional  smaller  projection,  the  prominentia  spiralis,  or 
accessory  spiral  ligament,  distinguished  usually  by  the  presence 

Fig.  401. 


Section  of  single  turn  of  cat's  cochlea :  SV,  SM,  ST,  scala  vestibuli,  media,  and  tympani ; 
a,  osseous  tissue  projecting  as  spiral  lamina  (b)  ;  c,  basilar  membrane  attached  to  spiral  liga- 
ment (d)  on  outer  wall ;  e,  concave  surface  lined  by  flat  cells  {g)  interrupted  by  spiral  promi- 
nence (_/")  containing  blood-vessel  ;  g,  stria  vascularis;  h,  Reissner's  membrane  covered  by 
epithelium  (t)  of  cochlear  duct  and  by  endothelium  («)  of  scala  vestibuli  ;  i,  limbus  from 
which  extends  membrana  tectoria  (k)  overhanging  Corti's  organ  ;  in,  tunnel  of  Corti ;  r,  s, 
inner  and  outer  hair  cells  ;  /,  cells  of  Claudius  ;  «,  spiral  ganglion  ;  o,  nerve-bundles  ;  v, 
blood-vessel. 

of  several  small  blood-vessels.  The  part  of  the  wall  lying  be- 
tween this  prominence  and  the  point  of  attachment  of  Reissner's 
membrane  is  occupied  by  a  peculiar  vascular  structure,  the  stria 
vascularis. 

The  epithelium  covering  the  outer  wall  of  the  cochlear  duct 
varies  in  different  positions  ;  the  usual  low  flat  cells  become  higher 
and  more  cuboidal  within  the  area  corresponding  to  the  stria  vas- 
cularis ;  over  the  prominence  the  cells  again  become  flat  and  poly- 
hedral, but  increase  in  height  on  approaching  the  basilar  membrane. 

The  stria  vascularis  is  remarkable  on  account  of  the  existence 
of  capillary  blood-vessels  within  an  epithelial  structure.  The 
presence  of  numerous  vessels  within  the  area  is  readily  established, 


THE  ORGAN   OF   HEARING.  ?gl 

likewise  the  undoubted  epithelial  character  of  the  innermost  cells 
next  the  endolymph,  but  uncertainty  exists  concerning  the  true 
nature — whether  epithelium  or  endothelium — of  the  cells  filling  the 
intercapillary  spaces  and  lying  between  the  epithelial  layer  and  the 
adjacent  connective  tissue. 

The  tympanic  wall  of  the  cochlear  duct  consists  of  two  por- 
tions,— the  limbus,  which  includes  the  wall  from  the  attachment 
of  Reissner's  membrane  to  the  end  of  the  lamina  spiralis,  and  the 
basilar  membrane,  which  extends  from  the  end  of  the  bony 
spiral  lamina  to  the  basilar  crest  on  the  outer  wall. 

The  limbus  corresponds  to  a  conspicuous  local  increase  in  the 
periosteum  and  the  fibrous  coat  at  the  point  where  the  apical  border 
of  the  cochlear  duct  is  attached  to  the  bony  spiral  lamina.  The 
greatest  thickening  of  the  periosteal  tissue  occurs  within  the  half 
of  the  limbus  next  the  membrane  of  Reissner,  the  half  adjoining 
the  basilar  membrane  exhibiting  an  abrupt  decrease  in  the  layer, 
marked  by  a  sharp  edge  overhanging  the  sulcus  spiralis,  the  con- 
cavity formed  by  the  receding  border  of  the  suddenly-diminished 
stratum ;  the  upper  and  lower  edges  of  the  recess  constitute  the 
superior  and  inferior  labia. 

The  thicker  portion  of  the  limbus  between  the  membrane  of 
Reissner  and  the  superior  labium  is  remarkably  modelled,  since 
its  surface  is  broken  by  clefts  and  furrows,  which  become  deeper 
as  well  as  larger  towards  the  margin  of  the  superior  labium  ;  this 
peculiar  arrangement  culminates  in  the  deeply-cleft  edge  of  the 
superior  labium,  where  irregular  tongue-like  processes  separated  by 
lateral  clefts  form  the  so-called  auditory  teeth,  the  entire  number 
of  which  in  the  human  cochlea  has  been  estimated  at  about  2500. 

The  epithelium  covering  the  limbus  differs  in  various  parts  ; 
flat  polyhedral  cells  cover  the  elevated  portions,  including  the 
auditory  teeth,  the  intervening  furrows  and  clefts  being  clothed  by 
columnar  elements.  The  epithelium  lining  the  sulcus  spiralis 
consists  of  a  single  layer  of  low  cuboidal  or  flattened  polyhedral 
cells  continuous  with  the  investment  of  the  auditory  teeth  on  the 
one  hand  and  with  the  highly-specialized  elements  of  Corti's  organ 
on  the  other. 

The  basilar  membrane,  the  outer  zone  of  the  tympanic  wall, 
stretches  from  the  end  of  the  osseous  spiral  lamina  to  the  basilar 
crest  of  the  spiral  ligament  of  the  outer  wall.  The  membrane  bears 
upon  part  of  its  surface  directed  towards  the  cochlear  duct  the  re- 
markably modified  neuro-epithelium  constituting  the  organ  of 
Corti,  and  is  consequently  divided  into  the  inner  zona  tecta,  over 
which  this  end-organ  lies,  and  the  outer  zona  pectinata,  covered 
with  the  more  usual  epithelium. 


392 


NORMAL    HISTOLOGY. 


The  basilar  membrane  includes  three  distinct  layers, — the  epithe- 
lium, the  substantia  propria,  and  the  tympanic  lamella.  The 
substantia  or  membrana  propria  consists  of  almost  homoge- 
neous connective  tissue,  and  represents  an  enormously-developed 
basement-membrane  beneath  the  highly-specialized  epithelium  of  the 
tympanic  wall.  This  stratum  is  covered  by  a  layer  of  peculiar  con- 
nective tissue,  the  tympanic  lamella,  directly  continuous  with  the 
tympanic  periosteum. 


The  lamella  contains   numbers 
character  interspersed  with  fibres  ; 


h 


m 


of  fusiform  cells  of  immature 
in  this  position  the  differentiation 
of  the  mesodermic  cells 
lining  the  tympanic 
canal  has  never  ad- 
vanced to  the  produc- 
tion of  typical  endo- 
thelial plates,  the  free 
surface  of  the  lamella 
being  invested  by  the 
short  fusiform  cells 
alone. 

The  epithelium 
covering  the  basilar 
membrane  within  the 
inner  zone  forms  the 
remarkable  organ  of 
Corti,  the  highest  ex- 
ample of  specialization 
of  neuro  -  epithelium 
anywhere  encountered. 
The  organ  of  Corti  extends  the  entire  length  of  the  cochlear  duct, 
with  the  exception  of  a  short  distance  within  the  blind  terminal  sacs 
at  the  two  ends  of  the  canal,  where  the  neuro-epithelium  is  wanting. 
In  general  it  consists  of  a  series  of  epithelial  arches  formed  by 
the  interlocking  of  the  ends  of  two  converging  greatly  modified 
epithelial  cells,  the  pillars  or  rods  of  Corti,  upon  the  inner  and 
outer  sides  of  which  rest  groups  of  neuro-epithelium  ;  the  tri- 
angular space  included  between  the  converging  pillars  of  Corti  above 
and  the  basilar  membrane  below  constitutes  the  tunnel  of  Corti, 
which  is,  therefore,  only  an  intercellular  space  of  unusual  size, 
containing  probably  a  soft  semi-fluid  intercellular  substance  serving 
to  support  the  nerve-fibrils  traversing  the  space. 

Examined  in  detail,  the  pillars  or  rods  of  Corti  prove  to  be  com- 
posed of  two  parts,  the  denser  substance  of  the  pillar  proper 
and  a  thin  imperfect  protoplasmic   envelope,   which  presents  a 


'li.  ■£>- 


Section  of  Corti's  organ  from  guinea-pig's  cochlea  :  57",  scala 
tympani ;  TC,  tunnel  of  Corti ;  a,  bony  tissue  of  spiral  lamina  ; 
i,  fibrous  tissue  covering  same  continued  as  substantia  propria 
of  basilar  membrane  ;  c,  c' ' ,  protoplasmic  envelope  of  Corti's 
pillars  {e,  e1) ;  d,  endothelial  plates ;  /,  heads  of  pillars  contain- 
ing oval  areas  ;  g,  head-plates  of  pillars  ;  h,  hi ',  inner  and  outer 
hair-cells ;  »i,  membrana  reticularis ;  k,  I,  cells  of  Hensen  and 
of  Claudius;  n,  nerve-libres  ;   i,  cells  of  Deiters. 


THE    ORGAN    OF    HEARING.  393 

triangular    nucleated    thickening   at  the  base   directed  towards 
the  cavity  of  the  tunnel. 

Each  pillar  possesses  a  slender  slightly-S-shaped  longitudinally- 
striated  body,  whose  upper  end  terminates  in  the  triangular  head, 
and  whose  lower  extremity  expands  into  the  foot  resting  upon  the 
basilar  membrane.  The  inner  pillar  is  shorter,  more  perpendicular, 
and  less  curved  than  the  outer ;  its  head  exhibits  a  single  or  double 
concave  articular  facet  for  the  reception  of  the  corresponding 
convex  surface  of  the  head  of  the  outer  rod.  The  cuticular  sub- 
stance of  both  pillars  adjoining  the  articular  surfaces  is  distinguished 
by  a  circumscribed  seemingly  homogeneous  oval  area  of  different 
nature.  The  upper  straight  border  of  the  head  of  both  pillars  is 
prolonged  outwardly  into  a  thin  process  or  head-plate,  that  of  the 
inner  rod  lying  uppermost  and  covering  over  the  head  and  inner 
part  of  the  plate  of  the  outer  pillar  ;  the  head-plate  of  the  latter  is 
longer  and  projects  beyond  the  termination  of  the  plate  of  the  inner 


■//fff 


Diagrammatic  view  of  Corti's  organ  :  A  ,  inner  pillars  of  Cord  (with  head-plates  a)  ;  B,  outer  pillars  ; 
C,  tunnel  of  Corti ;  D,  hasilar  membrane ;  E ,  inner  hair-cells  ;  i,  i',  membrana  reticularis ;  2,  2',  1'' , 
rows  of  outer  auditory  hairs  projecting  between  phalanges  (4-4") ;  5,  terminal  plates ;  6-6",  outer 
hair-cells ;  7-7",  cells  of  Deiters  ;  8,  cells  of  Claudius.     (After  Testut.) 

rod  as  the  phalangeal  process,  uniting  with  the  adjacent  pha- 
langes of  the  cells  of  Deiters  to  form  the  membrana  reticularis. 
The  inner  pillars  of  Corti  are  more  numerous  but  narrower  than 
the  outer  elements,  from  which  arrangement  it  follows  that  the 
broader  outer  rods  articulate  with  two  and  sometimes  three  of  the 
inner  pillars,  the  number  of  the  latter  in  man  being  estimated  by 
Retzius  at  5600,  as  against  3850  of  the  outer  rods. 


394 


NORMAL    HISTOLOGY. 


Immediately  within  the  arch  of  Corti,  resting  upon  the  inner 
rods,  a  single  row  of  specialized  epithelial  elements  extends  as 
the  inner  hair-cells.  These  elements,  little  more  than  half  the 
thickness  of  the  epithelial  layer  in  length,  possess  a  columnar  body, 
whose  dark  granular  protoplasm  contains  an  oval  nucleus  ;  the 
outer  somewhat  constricted  end  of  the  cells  is  limited  by  a  sharply- 
defined  cuticular  zone,  from  the  free  surface  of  which  project,  in 
man,  some  twenty  fine  rods  or  "  hairs."  The  inner  hair-cells  are 
less  numerous,  as  well  as  shorter  and  broader,  than  the  correspond- 
ing outer  elements  ;  their  numerical  relation  to  the  inner  rods 
of  Corti  is  such  that  to  every  three  rods  two  hair-cells  are  applied. 

The  inner  sustentacular  cells  extend  throughout  the  thickness 
of  the  epithelial  layer,  and  exhibit  a  slightly-imbricated  arrangement 
as  they  pass  over  the  sides  of  Corti' s  organ  to  become  continuous 
with  the  lower  cells  of  the  sulcus  spiralis. 

The  cells  covering  the  basilar  membrane  from  the  outer  pil- 
lar to  the  basilar  crest  comprise  three  groups  :  those  composing  the 
outer  part  of  Corti's  organ,  including  the  outer  hair-cells  and  cells 
of  Deiters,  the  outer  supporting  cells,  or  cells  of  Hensen, 
and  the  low  cuboidal  elements,  the  cells  of  Claudius,  investing  the 
outermost  part  of  the  basilar  membrane. 

The  outer  hair-cells  are  far  more  numerous  than  the  corre- 
sponding inner  elements,  and  in  man  and  apes  are  disposed  in  three 
or  four  rows,  alternating  with  the  peculiar  end-plates  or  "pha- 
langes" of  Deiters's  cells  which  separate  the  ends  of  the  hair-cells 
and  join  to  form  a  cuticular  net-work,  the  membrana  reticularis, 
through  the  openings  of  which  the  hair-cells  reach  the  free  surface. 
The  inner  row  of  these  cells  lies  directly  upon  the  outer  rods  of 
Corti,  so  placed  that  each  cell,  as  a  rule,  rests  upon  two  rods  ;  the 
cells  of  the  second  row,  however,  are  so  disposed  that  each  cell  lies 
opposite  a  single  rod,  while  the  third  layer  repeats  the  arrangement 
of  the  first  ;  in  consequence  of  this  grouping  these  elements,  in  con- 
nection with  the  "phalanges,"  appear  in  surface  views  like  a  checker- 
board mosaic,  in  which  the  oval  free  ends  of  the  auditory  cells 
are  included  between  the  peculiar  compressed  and  indented  octag- 
onal areas  of  the  end-plates  of  Deiters's  cells. 

The  outer  auditory  or  hair  cells  are  cylindrical  in  their  general 
form,  terminating  about  the  middle  of  the  epithelial  layer  in  slightly- 
expanded  rounded  ends  near  which  the  spherical  nuclei  are 
situated.  The  outer  sharply-defined  ends  of  the  cells  are  distin- 
guished by  a  cuticular  border  supporting  about  twenty  fine,  rigid 
auditory  rods  or  "  hairs"  which  project  beyond  the  level  of  the 
membrana  reticularis. 

The  sustentacular  elements,  the  cells  of  Deiters,  have  much 


THE    ORGAN    OF    HEARING.  •jg- 

in  common  with  the  rods  of  Corti,  being  specialized  epithelial 
cells  which  extend  the  entire  thickness  of  the  epithelial  stratum  to 
terminate  in  the  peculiar  end-plates  or  phalanges.  It  follows  that 
while  the  free  surface  of  Corti' s  organ  is  composed  of  both  auditory 
cells  and  sustentacular  cells,  the  elements  resting  upon  the  basi- 
lar membrane  are  of  one  kind  alone, — the  cells  of  Deiters.  The 
bodies  of  the  latter  consist  of  two  parts, — the  elongated  cylindri- 
cal chief  portion  of  the  cell,  containing  the  spherical  nucleus 
and  resting  upon  the  basilar  membrane,  and  the  greatly-attenuated 
pyramidal  phalangeal  process.  A  system  of  communicating 
intercellular  clefts,  the  spaces  of  Nuel,  lies  between  the  auditory 
and  supporting  cells  ;  these  are  occupied  by  a  semi-fluid  inter- 
cellular substance,  like  the  tunnel  of  Corti,  which  they  connect. 

The  membrana  tectoria,  or  Corti's  membrane,  stretches  from 
the  upper  lip  of  the  limbus  above  the  sulcus  spiralis  and  Corti's  organ  as 
far  as  the  last  row  of  the  outer  hair-cells.  The  membrane  is  a  cutic- 
ular  production  originally  formed  by  the  cells  covering  the  region  of 
the  auditory  teeth  and  the  spiral  sulcus  ;  at  first  it  rests  upon  the 
epithelial  cells,  but  later  it  becomes  separated  from  those  lying  external 
to  the  free  edge  of  the  auditory  teeth  and  assumes  its  conspicuous  posi- 
tion over  the  organ  of  Corti.  The  membrane  seems  to  be  composed  of 
fine  resistant  fibres  held  together  by  an  interfibrillar  cement-sub- 
stance. During  life  the  membrane  is  probably  soft  and  gelatinous 
and  much  less  rigid  than  its  appearance  after  reagents  indicates. 

The  outer  sustentacular  cells,  or  cells  of  Hensen,  form  an 
outer  zone  immediately  external  to  the  last  Deiters' s  cell ;  these  ele- 
ments resemble  the  inner  sustentacular  cells,  but  differ  somewhat  in 
form  and  arrangement.  In  consequence  of  their  oblique  positions 
the  bodies  are  not  only  greatly  elongated  but  also  imbricated. 

The  cells  of  Claudius  are  the  direct  continuations  of  Hensen's 
cells,  and  pass  uninterruptedly  into  the  low  columnar  elements 
covering  the  remaining  part  of  the  basilar  membrane.  These  cells 
possess  clear  faintly  granular  protoplasm,  in  which  pigment- 
granules  are  frequently  seen  as  well  as  spherical  nuclei.  At  the 
outer  extremity  of  the  basilar  membrane  these  cells  are  continuous 
with  the  epithelium  covering  the  upper  surface  of  the  basilar  crest. 

The  nerves  of  the  cochlea,  branches  of  the  cochlear  division 
of  the  auditory  nerve,  present  an  intricate  arrangement,  the  exact 
mode  of  their  ultimate  termination  being  still  in  many  points  un- 
certain. With  the  exception  of  bundles  for  the  supply  of  the  first 
turn,  which  run  in  channels  leading  directly  to  the  peripheral 
spiral  canal,  the  cochlear  nerves  pass  into  the  central  canal  of  the 
modiolus,  from  which  a  series  of  large-sized  lateral  branches  diverges 
at  quite  regular  intervals   through   canals  communicating  with  the 


^g5  normal  histology. 

peripheral  spiral  canal  within  the  base  of  the  bony  spiral  lamina. 
Within  the  peripheral  canal  the  nerve-fibres  are  augmented  by 
numerous  nerve-cells,  continuing  along  the  spiral  canal  as  the 
ganglion  spirale.  From  this  numerous  twigs  are  given  off,  which 
pass  along  the  canals  within  the  spiral  lamina  towards  its  margin, 
the  twigs  meanwhile  subdividing  to  form  an  extensive  plexus  con- 
tained within  corresponding  channels  in  the  bone.  At  the  edge  of 
the  spiral  lamina  bundles  of  fine  fibres  are  given  off,  which  escape  at 
the  foramina  nervina  and  enter  the  epithelium  close  to  the  inner 
rod  of  Corti.  During  or  before  their  passage  through  the  foramina 
the  nerve-fibres  lose  their  medullary  substance  and  proceed  to 
their  destination  as  fine,  naked  axis-cylinders. 

The  radiating  bundles  pass  within  the  epithelium  to  the  inner 
side  of  the  base  of  the  inner  pillar,  where  they  divide  into  two 
sets  of  fibrillse,  one  going  to  the  inner  hair-cells,  the  other  passing 
between  the  inner  pillars  to  reach  the  tunnel.  After  crossing  this 
space  the  fibrillar  escape  between  the  outer  rods  into  the  epithe- 
lium lying  on  the  outer  side  of  the  arch.  The  further  course  of  the 
fibrillar  seems  to  be  such  that  some  fibrillse  extend  between  the 
outer  pillar  of  Corti  and  the  first  row  of  hair-cells,  while  succeeding 
groups  of  fibrillae  course  between  the  rows  of  Deiters's  cells  to  reach 
the  remaining  hair-cells.  The  exact  relation  between  the  nerve- 
fibrils  and  the  auditory  cells,  as  to  whether  the  fibrillae  actually  join 
the  cells  or  only  come  in  close  contact,  is  yet  a  matter  of  uncertainty, 
although  renewed  investigations  render  it  still  improbable  that  direct 
anatomical  continuity  exists. 

The  ductus  and  saccus  endolymphaticus  possess  walls  which 
closely  correspond  with  those  of  the  saccule  and  the  utricle,  com- 
posed of  a  thin  connective-tissue  tunica  propria  supporting  the 
lining  of  ectodermic  epithelium  ;  the  latter  consists  of  a  single 
layer  of  flat  polyhedral  cells.  The  duct  lies  within  the  bony 
aqueduct,  closely  united  with  the  periosteal  lining,  unsurrounded  by 
an  extension  of  the  perilymphatic  space  ;  in  a  few  localities  a  meagre 
layer  of  loose  connective  tissue  forms  a  less  intimate  bond  between 
the  periosteum  and  the  fibrous  coat  of  the  duct. 

The  cochlear  perilymphatic  spaces,  the  scalse  vestibuli  et 
tympani,  include  within  their  walls  the  same  tissues  that  bound 
similar  cavities  within  other  parts  of  the  internal  ear.  The  perios- 
teum of  the  bony  cochlea  constitutes  the  fibrous  tunic,  which  is 
usually  covered  on  the  surface  in  contact  with  the  enclosed  perilymph 
by  a  single  layer  of  endothelial  plates  ;  in  some  localities,  however, 
as  on  the  tympanic  surface  of  the  basilar  membrane,  the  lining  cells 
retain  their  primitive  mesodermic  character,  never  becoming  fully 
differentiated  into  endothelium. 


THE   ORGAN    OF    HEARING.  W7 

The  blood-vessels  supplying  the  cochlea  constitute  two 
groups, — the  branches  distributed  to  the  membranous  cochlea 
and  the  numerous  twigs  destined  for  the  bony  capsule.  The 
cochlear  branch  of  the  auditory  artery,  just  before  its  passage 
through  the  bony  wall,  divides  into  fifteen  to  twenty  twigs,  which 
pass  either  directly  through  canals  to  supply  the  lowest  turn  of  the 
cochlea  or  into  the  modiolus.  The  vessels  within  the  central  canal 
of  the  modiolus,  after  supplying  the  nerve-trunks  and  the  spiral 
ganglion  with  nutritive  twigs,  send  off  lateral  branches,  which  form 
two  remarkable  masses  of  coiled  vessels,  the  glomeruli  cochleae  ; 
from  the  larger  of  these,  situated  somewhat  above  the  point  of  origin 
of  the  bony  spiral  lamina,  arterioles  proceed  to  Reissner'  s  membrane 
and  to  the  limbus,  breaking  up  to  form  the  capillary  net-works  of 
these  structures.  The  smaller  glomeruli,  within  the  base  of  the 
partitions  separating  the  adjoining  cochlear  turns,  send  off  branches 
forming  two  independent  capillary  systems.  These  are  the  net-works 
within  the  basilar  membrane  and  those  of  the  stria  vascularis, 
which,  while  having  a  common  origin,  do  not  communicate.  The 
capillaries  of  the  membranous  cochlea  are  collected  into  two 
principal  trunks,  the  vas  prominens  on  the  outer  wall  and  the  vas 
spirale  beneath  the  basilar  membrane  opposite  the  inner  rods  of 
Corti ;  from  these  channels  the  blood  is  conveyed  to  the  larger  venous 
trunk,  the  vena  spiralis  modioli,  lying  below  the  spiral  ganglion 
within  the  base  of  the  osseous  spiral  lamina. 

The  lymphatics  of  the  internal  ear  are  represented  by  the  large 
lymph-spaces  included  between  the  membranous  labyrinth  and 
its  bony  capsule, — the  perilymphatic  spaces  of  the  semicircular 
canals,  the  utricle,  the  saccule,  and  the  cochlea.  These  large  inter- 
communicating spaces  are  in  direct  exchange  with  the  subarach- 
noidean  and  probably  also  the  subdural  intra-cranial  lymph-cav- 
ities. The  demonstrated  communication  between  the  cavity  of 
the  endolymph  and  the  subdural  space  by  means  of  the  saccus 
endolymphaticus  brings  the  contents  of  the  membranous  labyrinth 
into  closer  relations  with  the  lymphatic  system  than  was  formerly 
recognized. 

THE    DEVELOPMENT    OF    THE    EAR. 

The  development  of  the  ear  includes  the  formation  of  two  mor- 
phologically distinct  divisions,  the  membranous  labyrinth,  the 
essential  auditory  structure,  and  the  accessory  parts,  comprising 
the  middle  ear,  with  its  ossicles  and  associated  cavities,  and  the 
external  auditory  canal  and  the  pinna. 

The  developmental  history  of  the  organ  of  hearing  proper  in  its 
early  stages  is  largely  an  account  of  the  growth  and  differentiation 


^g3  normal  histology. 

of  the  ectodermic  otic  vesicle,  since  from  this  is  produced  the  im- 
portant membranous  tube,  the  enveloping  fibrous  and  osseous 
structures  being  comparatively  late  contri- 
Fig.  404.  butions  from  the  mesoderm. 

The  internal  ear  first  appears  as  a  thick- 
ening and  soon  after  depression  of  the 
ectoderm  within  a  small  area  on  either 
side  of  the  cephalic  end  of  the  neural  tube  at 
a  level  corresponding  to  about  the  middle 
of  the  future  medulla.  This  auditory  pit 
is  widely  open  for  a  considerable  time  and 
distinguished  by  the  great  thickness  of  its 
depressed  wall,  which  contrasts  strongly 
with  the  adjacent  ectoderm.  After  a  time 
the  lips  of  the  pit  approximate  until  by 
their  final  union  the  cup-like  depression 
is  converted  into  a  closed  sac,  the  otic 
vesicle. 

The  otic  vesicle,  after  severing  all  con- 
nection with  the  ectoderm,  gradually  re- 
cedes from  the  surface  in  consequence  of 
the  growth  of  the  intervening  mesodermic 
layer  ;  it  next  loses  its  spheroidal  form  and  becomes  pear-shaped, 
with  the  smaller  end  directed 
dorsally.  This  diverticulum 
is  the  first  appearance  of  the 
recessus  vestibuli,  a  divi- 
sion of  the  embryonal  laby- 

Fig.  405. 


^8'6 


Section  through  developing  ear 
of  nine-and-a-half-day  rabbit  em- 
bryo :  e,  ectoderm  thickened  and 
invaginated  to  form  auditory  pit 
a.t  o ;  m,  surrounding  still  undif- 
ferentiated mesoderm  ;  n,  lining 
of  neural  tube;  v,  blood-vessel. 


Sagittal  section  through  developing  ear 
of  ten-day  rabbit  embryo :  o,  otic  vesicle 
becoming  pear-shaped,  due  to  formation 
of  recessus  vestibuli  (r)  ;  nt,  surrounding 
mesoderm. 


Section  through  developing  ear  of  twelve-day 
rabbit  embryo  :  v,  primitive  vestibule,  from 
which  extend  (r)  recessus  labyrinthi  and  (s) 
semicircular  canal  above  and  (c)  cochlear  canal 
below  ;  n.  neural  tube  with  thickened  ventral 
lining  ;  m,  mesoderm. 


rinth  disproportionately  conspicuous  compared  with  its  permanent 
representative,  the  ductus  endolymphaticus. 


THE    ORGAN    OF    HEARING.  ,gg 

The  semicircular  canals  next  form  as  tubular  projections  from 
the  vesicle  and  rapidly  assume  great  prominence  ;  the  superior 
vertical  canal  appears  first,  and  the  external  or  horizontal 
canal  last.  The  growth  of  the  epithelial  diverticula  is  later  accom- 
panied by  a  condensation  of  the  surrounding  mesoderm,  which 
differentiates  into  an  external  layer,  the  future  cartilaginous  and 
later  bony  capsule,  and  an  inner  layer  of  fibrous  tissue.  The  latter 
suffers  partial  atrophy  and  absorption,  in  consequence  clefts  appear 
among  the  delicate  bundles,  an  arrangement  permanently  represented 
by  the  fibrous  walls  and  intervening  trabeculae  of  the  spaces  occu- 
pied by  the  perilymph  surrounding  the  membranous  canals.  Within 
the  ampullae,  which  early  develop,  the  epithelial  lining  undergoes 
specialization,  accompanied  by  thickening  of  the  mesodermic  wall 
within  circumscribed  areas  to  form  the  cristae  acusticae. 

Coincidently  with  the  development  of  the  semicircular  canals  a 
diverticulum — the  cochlear  canal — appears  at  the  lower  anterior 
end  of  the  membranous  sac  ;  this  tube,  oval  in  section,  grows  for- 
ward, downward,  and  inward,  and  represents  the  future  cochlear 
duct,  or  scala  media.  After  attaining  considerable  length,  further 
elongation  is  accompanied  by  coiling  and  the  assumption  of  the 
permanent  disposition  of  the  tube. 

The  epithelium  of  the  cochlear  tube  early  exhibits  a  distinction, 
the  cells  of  the  upper  surface  of  the  somewhat  flattened  canal  be- 
coming attenuated,  while  those  on  the  lower  wall  undergo  thick- 
ening and  further  differentiation ;  the  flattened  cells  form  the 
covering  of  Reissner's  membrane  and  the  outer  wall,  and 
the  taller  elements  are  converted  into  the  complicated  structures  of 
the  tympanic  wall  of  the  scala  media,  including  the  crista,  the 
sulcus,  and  the  organ  of  Corti. 

The  development  of  these  structures  includes  the  differentiation 
of  two  epithelial  ridges  ;  from  the  inner  and  larger  of  these  is 
derived  the  lining  of  the  sulcus  spiralis  and  the  overhanging  mem- 
brana  tectoria,  and  from  the  outer  and  smaller  ridge  is  produced 
the  elaborate  and  complicated  organ  of  Corti.  The  crista  appears 
between  the  sulcal  cells  and  the  cochlear  axis  as  a  thickening  of  the 
spiral  lamina. 

The  cochlear  outgrowth  of  the  primary  otic  vesicle  forms  the 
membranous  cochlea,  or  scala  media,  alone,  the  walls  of  the 
adjacent  divisions,  the  scala  vestibuli  and  scala  media,  resulting 
from  the  changes  within  the  surrounding  mesoderm.  The  latter 
differentiates  into  two  zones,  an  outer,  which  becomes  the  car- 
tilaginous, and  finally  osseous,  capsule,  and  an  inner,  lying 
immediately  around  the  membranous  canal,  which  for  a  time  consti- 
tutes a  stratum  of  delicate  connecting  tissue  between  the  denser 


400  NORMAL    HISTOLOGY. 

capsule  and  the  ectodermic  canal.  Within  this  layer  clefts  appear, 
which  gradually  extend  until  two  large  spaces  bound  the  mem- 
branous cochlea  above  and  below. 

These  spaces,  the  scala  vestibuli  and  the  scala  tympani,  are 
separated  for  a  time  from  the  scala  media  by  a  robust  septum 
consisting  of  a  mesodermic  layer  of  considerable  thickness  and  the 
wall  of  the  ectodermic  tube.      With  the  further  increase  in  the 

dimensions  of  the  lymph-spaces  the  par- 

".';  '"• '  titions  separating  them  from  the  cochlear 

q  ~  .       duct  are  correspondingly  reduced,  until, 

^•-^^  ..^      Nx  finally,  the  once  broad  layers  are  rep- 

"/">■-.-  -  resented  by  frail  and  attenuated  struct- 

.  -'     -  -  ures,  the  membrane  of  Reissner  and 

the   basilar    membrane,    which   con- 
•  .•  ;  __VL?y ;  •      sequently     include      an      ectodermic 

^<-'  .  /-.;  ,7/-.  .^A  ;/  -  stratum,  the  epithelial  layer,  strength- 
,£.  -?/_'■  .'""■..  I    '         ened  by  a  mesodermic  lamina,  rep- 

resented by  the  substantia  propria  and 
< -<L  its  endothelioid  covering. 

Section  through  developing  cochlea         The  main  sac  of  the  otic  vesicle  from 

of   twentv-one-day  rabbit   embryo:    e,  «•    i        .1  r  •  •■  .■       <■ 

sections  of  ectodermic  cochlear  duct,  or  whlch     the     foregoing     diverticula     arise 

scala  media,  surrounded  by  delicate  mes-  constitutes  the  primitive  meiTlbra- 
odermic  tissue  (>«),  within  which  large  *_•!_     1  1     1     .  1     1-     •  1 

lymph-spaces  later  appear;   c,  con-     nous  vestibule,  and   later  subdivides 

densed cartilaginous  capsule  ;n,  bundles  into  the  SaCCUle  and  the  Utricle. 
of  nerve-fibres.  ,-„.   .  ...  , 

lhis  separation  begins  as  an  annular 
constriction  of  the  primitive  vestibule,  incompletely  dividing  the 
vesicle  into  two  compartments ;  the  ductus  endolymphaticus 
unites  with  the  narrow  canal  connecting  these  vesicles  in  such  man- 
ner that  each  space  receives  one  of  a  pair  of  converging  limbs,  an 
arrangement  foreshadowing  the  permanent  relations  of  the  parts. 

Even  before  the  subdivision  of  the  primitive  vestibule  is  established 
the  vestibular  end  of  the  cochlear  canal  becomes  constricted,  so 
that  communication  between  this  tube  and  the  future  saccule  is  main- 
tained by  only  a  narrow  passage,  the  canalis  reuniens.  The  de- 
velopment of  the  maculae  acusticae  of  the  saccule  and  the  utricle 
depends  upon  the  specialization  of  the  epithelium  within  certain 
areas  associated  with  the  distribution  of  the  auditory  nerves.  The 
nerve-fibres  form  their  ultimate  relations  with  the  sensory  areas  by 
secondary  growth  into  the  epithelial  structures. 

From  the  foregoing  it  is  apparent  that  the  membranous  laby- 
rinth is  genetically  the  oldest  part  of  the  internal  ear,  and  that  it 
is  in  fact  only  the  greatly  modified  and  specialized  closed  otic  vesi- 
cle surrounded  by  secondary  mesodermic  tissues  and  spaces. 

The  middle   ear  is  derived  from  the   expanded  and  metamor- 


THE    ORGAN    OF    HEARING. 


40I 


phosed  outer  end  of  the  first  pharyngeal  pouch,  or  inner  visceral 
furrow,  the  Eustachian  tube  representing  the  inner  segment. 
The  tympanic  membrane  results  from  the  changes  affecting  the 
septum  between  the  outer  and  inner  first  visceral  furrows  ;  this  par- 
tition, originally  thick,  consists  of  a  mesodermic  middle  stratum. 
covered  on  its  outer  and  inner  surfaces  respectively  by  the  ecto- 
derm and  the  entoderm.  The  external  and  the  middle  ear  at  no 
time  communicate,  but  are  normally  separated  by  the  septum  in 
question. 

The  ear-ossicles  are  developed  in  connection  with  the  primitive 
skeleton  of  the  visceral  arches  ;  the  malleus  and  the  incus  rep- 
resent specialized  parts  of  the  cartilaginous  rod  of  the  first  arch. 
The  development  of  the  stapes,  on  the  contrary,  is  probably  not 
connected  with  the  visceral  skeleton,  but  owes  its  formation  to  the 
ossification  of  the  tissues  at  the  fenestra  ovalis. 

The  development  of  the  external  ear  results  from  the  changes 
taking  place  within  the  first  outer  visceral  furrow,  or  gill-cleft, 
and  the  tissue  immediately  around  its  external  orifice.  From 
the  gill-cleft  originates  the  external  auditory  canal,  and  from  the 
margins  of  its  orifice  the  pinna  is  formed. 


26 


402 


NORMAL   HISTOLOGY. 


CHAPTER    XIX. 


THE  NASAL  MUCOUS  MEMBRANE. 

The  mucous  membrane  lining  the  nasal  fossae  consists  of  two  prin- 
cipal divisions,  that  of  the  respiratory  and  that  of  the  olfactory 
region ;  the  latter  alone  is  concerned  in  the  sense  of  smell. 

The  mucous  membrane  of  the  respiratory  region  is  dis- 
tinguished from  that  of  the  olfactory  area  by  its  thickness,  over  the 

inferior  turbinals  it 
reaching  4  mm., 
and  by  the  presence 
of  venous  net- 
works of  such  size 
that  the  structure 
appears  as  if  com- 
posed of  cavernous 
tissue.  The  epi- 
thelium of  this 
region  is  stratified 
ciliated  columnar 
in  type,  within  the 
superficial  layer  of 
which  numerous 
goblet-cells  are 
interspersed.  The 
tunica  propria  of 
this  region  is  composed  of  fibrous  connective  tissue  containing  clefts 
occupied  by  many  leucocytes,  the  latter  frequently  invading  the 
superimposed  epithelium  ;  occasional  nodules  of  lymphoid  tissue 
are  also  encountered  in  various  parts  of  the  mucosa.  The  surface 
of  the  tunica  propria  is  smooth,  since  the  usual  subepithelial  papillae 
are  here  wanting. 

The  mucous  membrane  of  the  respiratory  region  is  further  dis- 
tinguished by  numerous  small  racemose  glands,  which  open  on 
the  free  surface  by  funnel-like  pits,  readily  recognized  by  the  unaided 
eye,  and  lined  for  some  distance  by  epithelium  corresponding  to  that 
of  the  adjacent  surface.  These  glands  are  mixed  in  character, 
since  some  acini  secrete  serous  fluids,  while  others  elaborate 
mucous  products.  The  glandular  structures  occur  with  especial 
frequence  over  the  inferior  turbinated  bones,  although  on  the  lateral 


Section  through  mucous  membrane  of  respiratory  region  of  child's 
tn>se:  a,  ciliated  epiihelium  ;  /',  tunica  propria;  c,  submucous  con- 
nective tissue  ;  d,  mucous  glands;  e,  duct  of  glands  opening  on  free 
surface  ;  /,  b.ood-vessels. 


THE  NASAL  MUCOUS  MEMBRANE.  40} 

walls  and  on  the  lower  part  of  the  nasal  septum  they  are  present  in 
large  numbers.  As  already  mentioned,  the  veins  of  the  mucosa  are 
so  wide  and  plentiful  that  the  layer  in  which  they  lie  appears  like 
cavernous  tissue. 

The  mucous  membrane  lining  the  accessory  spaces  of  the  nasal 
fossae — the  sphenoidal,  the  frontal,  and  the  maxillary  sinuses 
and  the  ethmoidal  cells — closely  resembles  that  of  the  respiratory 
region,  being  covered  by  a  stratified  ciliated  epithelium,  which 
rests  upon  a  thin  tunica  propria  closely  united  with  the  periosteum. 
These  tracts  are  chiefly  distinguished  from  the  respiratory  surface  by 
the  marked  reduction  in  the  thickness  of  the  mucous  membrane, 
which  within  these  spaces  is  seldom  more  than  .02  mm.  The  meagre 
supply  of  glands  is  another  point  of  difference,  the  glandular  struct- 
ures within  these  spaces  being  represented  by  small  and  sparingly 
distributed  groups  often  exhibiting  peculiar  modifications  of  the 
racemose  type. 

The  olfactory  region  is  distinguished  macroscopically  from  the 
respiratory  portions  of  the  nasal  fossae  by  the  yellowish-brown 
tinge  of  its  mucous  membrane  as  contrasted  with  the  rosy  hue  of 
that  covering  the  adjacent  region  ;  the  more  deeply  colored  tract 
indicates  in  only  a  general  way  the  boundaries  of  the  olfactory 
region,  since  the  limits  of  the  two  do  not  closely  correspond,  the 
brownish  area  in  man  being  usually  somewhat  less  extensive  than 
the  entire  surface 
possessing  the 
structure  of  the  ol- 
factory mucous 
membrane.  The 
latter  is  also  dis- 
tinguished by  its 
greater  thickness 
and  by  the  absence 
of  ciliated  cells. 

The  olfactory 
mucosa  consists  of 
the  epithelium  and 
the  tunica  propria, 
the  characteristic 
appearances  of  the 
tissue  depending 
upon  the  peculiari- 
ties of  the  former,  another  example  of  neuro-epithelium.  This  epi- 
thelium consists  of  two  kinds  of  cells,  the  sustentacular  and  the 
olfactory  elements.    The  sustentacular  or  support  cells  present 


Section  through  olfactory  mucous  mcmurane  from  child's  nose  :  a, 
zone  of  oval  nuclei  belonging  to  sustentacular  cells;  b,  zone  of 
spherical  nuclei  of  olfactory  elements  ;  c,  basilar  cells ;  d,  subepi- 
thelial tissue  ;  e,  glands  of  Bowman. 


404 


NORMAL   HISTOLOGY. 


an  outer  cylindrical  division,  containing  an  oval  nucleus,  situated 
always  near  the  inner  end  of  the  more  expanded  part  of  the  cell,  and 
yellowish  pigment,  together  with  numerous  granules  arranged 
more  or  less  markedly  in  longitudinal  rows.  The  outer  ends  of  the 
supporting  cells  are  modified  into  a  cuticular  zone,  the  membrana 
limitans  olfactoria,  sometimes  exhibiting  vertical  markings. 

The  nuclei  of  the  sustentacular  cells  form  a  regular  band,  the 
zone  of  oval  nuclei,  which  lies  next  the  free  surface,  and  strongly  con- 
trasts with  the  adjoining  broad  zone  of  round  nuclei  of  the  olfac- 
tory cells.  The  inner  portions  of  the  sustentacular  elements  ar^ 
very  narrow,  irregular  in  outline,  and  terminate  generally  in  cleft  or 
branched  processes  in  close  relation  with  the  underlying  basal  cells. 

The  olfactory  cells  lie  among  the  supporting  elements  as  incon- 
spicuous, elongated,  and  attenuated  bodies,  whose  variously-placed 
spherical  nuclei,  covered  by  a  thin  stratum  of  protoplasm,  consti- 
tute the  widest  parts  of  the  cells  ;  in  consequence  these  elements 
appear  like  spherules  from  the  outer  and  inner  poles  of  which  thin 
rod-like  processes  extend  towards  the  free  surface  and  the  base- 
ment-membrane. The  nuclei  of  the  olfactory  cells  lie  at  all  levels, 
forming  the  broad  zone  of  round  nuclei. 

The  deepest  part  of  the  epithelial  stratum  is  made  up  of  a  closely- 
set  zone  of  small  nucleated  cells,  resting  upon  the  tunica  propria 
on  the  one  hand,  and  sending  irregularly-branched  processes  among 
the  overlying  elements  on  the  other.  These  basilar  cells  consti- 
tute a  protoplasmic  net- work,  whose  extensions  and  continuities  are 
at  present  inadequately  determined. 

The  tunica  propria  of  the  olfactory  region  consists  of  a  moder- 
ately loose  felt-work  of  bundles  of  fibrous  connective  tissue,  inter- 
mingled with  numerous  delicate  elastic  fibres.  The  outermost  zone 
of  the  tunica  propria  is  condensed  to  form  a  very  slightly  developed 
basement-membrane,  upon  which  rests  the  epithelium.  Em- 
bedded within  the  mucosa  branched  tubular  glands,  or  Bowman's 
glands,  exist  in  great  abundance  ;  these  structures  possess  a  duct  of 
sufficient  length  to  extend  through  the  epithelial  layer,  the  remaining 
portions  of  the  tube  constituting  the  body  and  fundus  of  the  gland. 
The  epithelial  cells  lining  the  secreting  part  of  the  tube  contain 
brownish  pigment,  which  aids  in  producing  the  characteristic  color 
of  the  olfactory  mucous  membrane.  Although  formerly  regarded  as 
serous  in  type,  it  is  probable  that  Bowman's  glands  must  be  included 
within  the  mucous  group.  These  glands,  which  in  places  consti- 
tute an  almost  continuous  layer  of  secreting  tissue,  are  much  more 
generously  distributed  than  those  within  the  respiratory  region. 

The  blood-vessels  supplying  the  nasal  mucous  membrane  are 
especially  distinguished  by  the  size  and  profusion  of  the  veins. 


THE    NASAL   MUCOUS    MEMBRANE.  aqc 

The  arterial  stems  lie  in  the  deeper  layers  of  the  tunica  propria,  from 
which  twigs  are  sent  into  the  more  superficial  stratum,  where,  imme- 
diately beneath  the  epithelium,  a  subepithelial  capillary  net-work 
is  formed  ;  other  twigs  break  up  into  capillaries  which  surround  the 
glands.  The  veins  are  remarkable  for  their  size  and  number,  in 
many  places,  particularly  over  the  posterior  part  of  the  inferior  tur- 
binated bone,  giving  to  the  entire  tunica  propria  the  character  of 
cavernous  tissue. 

The  lymphatics  are  represented  by  numerous  vessels  which  con- 
stitute a  net-work  within  the  deeper  parts  of  the  tunica  propria  and 
around  the  lymphoid  nodules  ;  in  addition  to  these,  within  the  olfac- 
tory region  wide-meshed  net-works  of  perineurial  lymph-chan- 
nels extend  throughout  the  mucosa  of  the  olfactory  region. 
The  olfactory  mucous  membrane  is  further  provided  with  a  rich  sys- 
tem of  intercommunicating  lymph-spaces  within  the  groundwork 
of  the  tunica  propria,  which  empty  into  the  larger  lymphatic  net- 
works of  the  deeper  layers. 

The  nerves  of  the  nasal  mucous  membrane  are  of  two  kinds, 
those  providing  common  sensation  and  those  concerned  in  the 
special  sense  of  smell ;  the  relations  of  the  latter  with  the  neuro- 
epithelium  are  of  much  interest,  but  at  the  present  time  by  no  means 
definitely  determined.  The  larger  filaments  of  the  olfactory  nerve 
lie  against  the  bony  walls,  partially  embedded  within  corresponding 
grooves,  and  give  off  smaller  arching  bundles,  which  pass  within  the 
mucous  membrane  towards  its  epithelial  surface.  The  twigs,  even 
within  the  mucosa  in  many  places,  are  enclosed  by  perineurial 
sheaths  prolonged  from  the  intercranial  investment  of  the  olfactory 
nerve.  The  fibres  of  the  latter  are  without  the  medullary  sub- 
stance, being  bundles  of  the  axis-cylinder  fibrillae  enclosed  within 
the  neurilemma ;  on  reaching  the  epithelium  the  fibres  break  up  into 
their  component  fibrillae,  which  pass  as  naked,  often  varicose,  axis- 
cylinders  between  the  elements  of  the  neuro-epithelium.  It  is 
highly  probable  that  the  nerve-fibrillae  come  into  close  contact,  even 
if  not  into  actual  continuity,  with  the  inner  ends  of  the  olfactory  cells ; 
whatever  the  exact  actual  relation,  a  very  intimate  relation  between 
the  nerves  and  percipient  elements  may  be  assumed.  Additional 
twigs  from  the  trifacial,  composed  of  medullated  fibres,  are  also 
distributed  to  the  olfactory  region,  without,  however,  coming  in 
relation  with  the  olfactory  cells. 

The  development  of  the  nasal  mucous  membranes  proceeds 
from  the  ectoderm,  the  earliest  indication  of  these  structures  ap- 
pearing as  the  olfactory  plates,  two  areas  of  thickened  ectoderm 
immediately  above  the  primitive  oral  cavity  and  in  contact  with  the 
wall  of  the  fore-brain. 


406 


NORMAL    HISTOLOGY. 


The  olfactory  plates  are  converted  into  the  nasal  pits  by  the 
growth  and  elevation  of  the  surrounding  parts  on  all  sides  except 
the  under  surface,  along  which  the  nasal  pits  for  a  time  directly  com- 
municate with  the  primary  mouth.  In  addition  to  the  differentiation 
of  the  surrounding  tissues  into  the  structures  of  the  external  nose, 
the  close  relation  of  the  primary  nasal  surface  with  the  brain-vesicle 
disappears  with  the  changes  of  position  produced  by  the  develop- 
ment of  the  fore-brain  and  growth  of  the  tissues  forming  the  cranial 
case,  particularly  the  development  of  the  olfactory  ganglion  from 
the  olfactory  plate. 

The  complicated  surfaces  of  the  nasal  fossae  are  due  primarily  to  the 
appearance  of  the  superior,  middle,  and  inferior  turbinal  folds  on 
the  lateral  wall  of  the  nasal  recess.  Each  fold  comprises  the  dupli- 
cation of  ectoderm  enclosing  a  core  of  mesoderm ;  the  latter 
becomes  the  turbinal  cartilages  and  finally  the  corresponding 
osseous  plates.  The  differentiation  of  the  olfactory  region  from 
the  general  lining  of  the  nasal  fossae  takes  place  coincidently  with  the 
growth  of  the  olfactory  nerve-fibres  ;  the  details  of  the  histogenesis 
of  these  structures,  however,  are  still  but  imperfectly  determined. 


The  special  organs  of  taste  and  of  touch,  including  the  taste- 
buds  and  the  tactile  corpuscles,  have  been  already  considered  in 
connection  respectively  with  the  tongue  and  with  the  peripheral 
nerve-endings  and  the  skin. 


appendix: 

INCLUDING   THE   MOST   USEFUL   HISTOLOGICAL 
METHODS. 


The  advances  made  during  the  last  few  years  in  the  preparation 
of  tissues  for  microscopical  examination  have  been  so  important,  that 
no  one  proposing  to  undertake  practical  histological  investigations, 
normal  or  pathological,  can  afford  to  ignore  methods  of  work  which, 
although  somewhat  exacting,  yield  results  far  superior  to  the  older 
processes.  With  the  increased  facilities  for  producing  thoroughly 
good  and  accurate  preparations  the  current  standard  of  excellence 
has  advanced,  and  results  formerly  viewed  with  complacency  are  now 
often  regarded  as  incomplete  and  correspondingly  unsatisfactory. 

The  activity  of  the  last  half-dozen  years  has  resulted  in  greatly 
multiplying  the  details  of  histological  technology,  since  each  worker 
determines  what  new  procedures  or  modifications  of  existing  methods 
are  advantageous  for  his  own  special  purposes.  Useful,  and  for  the 
advanced  worker  indispensable,  summaries  of  the  ever-increasing 
methods  of  microscopical  investigation  are  to  be  found  in  the  volumes 
especially  devoted  to  technology  ;  of  such  works  in  English  the  excel- 
lent "  Microtomist's  Vade-Mecum,"  by  Bolles  Lee,  may  be  recom- 
mended, supplemented  by  the  notices  of  new  methods  presented  in 
the  current  issues  of  the  Journal  of  the  Royal  Microscopical  Society. 

In  the  present  place,  however,  no  attempt  will  be  made  to  discuss 
even  incompletely  many  of  the  methods  finding  use  at  the  hands  of 
various  investigators  ;  but,  on  the  contrary,  only  a  few  processes  will 
be  described  which  extended  use  has  proved  to  be  thoroughly  trust- 
worthy and  satisfactory.  The  student  undertaking  such  work  for  the 
first  time  is  strongly  advised  to  persevere  with  the  paraffin  method, 
as  here  described,  since,  when  properly  employed,  it  may  always  be 
depended  upon  to  yield  the  most  gratifying  results.  Failures,  sure 
to  beset  the  beginner,  should  be  carefully  analyzed  and  be  made  to 
yield  the  experience  which  will  guard  against  their  repetition. 

The  manipulations  necessary  for  the  conversion  of  the  fresh  tissue 
into  the  finished  preparation  are  : 

i.   Fixation  of  the  tissue. 

2.   Preservation  of  the  fixed  tissue. 

407 


408  APPENDIX. 

3.  Staining  of  the  tissue  in  toto. 

4.  Embedding  in  paraffin. 

5.  Sectioning. 

6.  Fixing  sections  to  the  slide. 

7.  Mounting. 

8.  Finishing,  labelling,  and  storing. 

1.  Fixation  of  the  Tissue.  By  "fixation"  is  understood  the 
killing  of  the  tissue  so  rapidly  that  its  elements  are  retained  exactly 
as  they  were  while  living  when  first  met  by  the  fixing  reagent  ;  thus, 
for  example,  extended  cells  should  remain  extended  after  death,  or 
rapidly-effected  changes,  as  those  of  karyokinesis,  should  be  retained 
in  the  stage  in  which  first  encountered,  and  not  be  allowed  to  com- 
plete their  cycle,  and  consequently  disappear,  as  when  the  tissue 
slowly  dies.  It  is  evident  that  absolutely  fresh  and,  for  many  investi- 
gations, still  living  tissues  are  essential  for  satisfactory  results  where 
the  condition  of  the  cells  is  a  matter  of  importance,  as  in  the  study 
of  the  structure  of  the  nucleus  or  of  the  protoplasm. 

While  so  evidently  desirable,  the  fulfilment  of  this  condition  in  the 
case  of  human  tissues  is  often  a  matter  of  impossibility,  or,  at  best, 
of  extreme  difficulty,  the  restrictions  imposed  upon  immediate  autop- 
sies rendering  it  usually  almost  impossible  to  secure  the  more  delicate 
tissues  while  their  cells  are  still  alive.  Fortunately,  however,  for  the 
majority  of  investigations,  the  exact  condition  of  the  cell  is  a  matter 
of  less  moment  than  its  general  form  and  its  relations  to  the  surround- 
ing elements  ;  for  such  purposes  the  slow  death  of  the  cells  may 
work  no  serious  detriment  to  the  usefulness  of  the  tissue,  but  it  is  to 
be  accepted  as  a  histological  maxim,  that  the  fresher  the  tissue  a?id 
the  more  acairate  the  fixation  of  its  elements,  the  more  valuable  a?id 
satisfactory  will  be  the  preparation. 

When,  then,  really  fresh  material  is  to  be  prepared  for  subsequent 
histological  examination,  it  is  to  be  subjected,  without  previous  wash- 
ing in  water,  first  to  the  action  of  some  fixation  fluid ;  the  choice 
of  the  reagent  to  be  employed  must  be  determined  by  the  purposes 
in  view  and  the  character  of  the  tissue. 

a.  Midler's  Fluid. 

Potassium  bichromate 2.5  gm. 

Sodium  sulphate i.ogm. 

Water 100  c.c. 

This  fluid,  when  properly  employed,  is  probably  the  most  generally 
useful  fixation  reagent ;  for  successful  results,  however,  strict  attention 
to  the  manner  of  its  employment  is  imperative.  The  quantity  of 
fluid  must  always  be  largely  in  excess  of  the  volume  of  the  tissue 


APPENDIX.  409 

treated,  and  the  tissue  should  not  be  in  pieces  over  2  cm.  in  thickness  ; 
the  fluid  should  be  changed  as  soon  as  it  becomes  turbid,  sometimes 
within  the  first  hour,  and  subsequently  renewed  as  often  as  may  be 
necessary  to  maintain  perfect  transparency .  Tissues  are  usually 
allowed  to  remain  in  Muller's  fluid  for  a  considerable  time,  two  weeks 
being  the  minimum,  while  they  may  be  permitted  to  lie  much  longer, 
usually,  without  disadvantage  ;  it  is  advisable,  however,  to  remove 
specimens  after  six  weeks,  and  preserve  them  in  spirit. 

The  tissue  is  transferred  from  the  Muller's  fluid  to  water,  and  thor- 
oughly washed  in  the  running  stream  from  4  to  6  hours,  until  all 
excess  of  the  fluid  has  been  removed  ;  it  is  then  placed  in  yo  per  cent. 
alcohol  and  kept  in  the  dark,  the  spirit  being  renewed  whenever 
strongly  tinged  by  the  removed  fluid  ;  as  long  as  discoloration  occurs 
an  occasional  change  of  alcohol  is  desirable. 

Where  the  interstitial  methods  of  embedding  are  followed,  no  great 
amount  of  hardening  is  necessary  or  even  desirable,  in  which  case 
the  tissues  are  best  stored  in  80  per  cent,  spirit,  where  they  may  lie 
until  needed.  Portions  of  the  nervous  system  which  are  subsequently 
to  be  stained  after  the  Weigert  process  may  be  fixed  with  advantage 
in  warm  Midler 's  fluid,  being  kept  in  an  oven  from  8  to  10  days  at 
a  temperature  of  350  C. 

b.  Absolute  Alcohol.  For  glands,  skin,  blood-vessels,  etc., 
absolute  alcohol  affords  a  rapid  and  admirable  means  of  fixation,  and 
possesses  the  additional  quality  of  simultaneously  hardening  the  tissue, 
a  matter  sometimes  of  great  convenience,  since  the  specimen  may  be 
cut  within  24  hours.  Small  pieces  of  tissue,  so  placed  either  by  sus- 
pension or  support  on  cotton  that  they  do  not  come  in  contact  with  the 
bottom  or  the  sides  of  the  glass  (to  which  they  otherwise  adhere),  are 
treated  from  12  to  24  hours,  the  alcohol  being  invariably  changed 
at  the  end  of  the  first  three  hours,  whether  cloudy  or  not.  After 
fixation  the  tissue  is  preserved  in  80  per  cent,  spirit.  It  is  to  be  noted 
that  the  action  of  95  per  cent,  alcohol  is  entirely  different  from  that 
of  the  absolute,  with  the  weaker  spirit  the  shrinkage  being  great  and 
the  fixation  imperfect ;  it  cannot,  therefore,  be  substituted. 

c.  Flemming's  Solution. 

Chromic  acid  (one-per-cent.  solution) 7.25  c.c. 

Osmic  acid  (two-per-cent.  solution) 2.50  c.c. 

Glacial  acetic  acid,  at  least 25  c.c. 

Where  the  structure  of  the  protoplasm  or  the  nucleus  is  to  be  in- 
vestigated, or  where  for  any  purpose  an  accurate  picture  of  the  cells 
is  desirable,  Flemming's  stronger  solution  (given  above)  will  be  found 
the  most  trustworthy  reagent  at  our  command.     Two  drawbacks  limit 


aIQ  APPENDIX. 

its  use  :  its  very  limited  power  of  penetration,  which  necessitates  the 
tissue  being  cut  in  layers  not  over  2-3  mm.  thick,  and  the  consider- 
able expense  attending  the  use  of  large  quantities  of  the  fluid.  The 
mixture  should  be  made  up  each  time  Just  before  using,  and  cannot 
be  employed  a  second  time.  The  living  tissue  is  placed  within  the 
solution  in  a  glass-stoppered  bottle,  and  allowed  to  remain,  without 
changing,  24  hours  ;  then  transferred  to  naming  water  1-2  hours, 
after  which  it  is  placed  in  70  per  cent,  alcohol,  and,  after  several 
changes,  preserved  in  80  per  cent,  spirit. 

d.  Picro-Sulphuric  Acid  (Kleinenberg's)  Solution. 

Picric  acid,  saturated  watery  solution 200  c.c. 

Sulphuric  acid,  pure 4  c.c. 

resulting  in  dense  precipitate  ;  after  one  hour  filter,  and  dilute  with 
three  volumes  (600  c.c.)  of  distilled  water. 

This  solution  is  an  admirable  and  trustworthy  reagent  for  embryos 
and  other  delicate  structures,  its  principal  objection  being  the  time 
required  to  remove  the  yellow  tinge  of  the  picric  acid.  The  embryos 
are  placed  directly,  without  washing,  into  the  fluid,  where  they  remain 
5  hours — if  very  large  the  time  may  be  extended  to  10—12  hours, 
with  a  renewal  of  the  fluid  ;  they  are  then  transferred  to  70  per  cent, 
alcohol,  which  is  repeatedly  changed  until  discoloration  no  longer 
takes  place  ;  preserve  in  80  per  cent,  alcohol. 

2.  Preservation  of  Tissues.  In  connection  with  fixation,  the 
subsequent  preservation  of  tissues  in  70  per  cent,  spirit  has  been  indi- 
cated ;  when,  however,  the  condition  of  the  specimen,  as  when  ob- 
tained some  time  after  death,  or  other  considerations,  render  fixation 
useless,  it  becomes  necessary  to  preserve  the  tissue  from  further  change. 
To  this  end  Muller's  fluid  may  also  be  advantageously  employed, 
observing  the  precautions  already  pointed  out,  followed  after  some 
weeks  by  alcohol.  In  many  cases,  however,  when  fixation  is  no 
longer  possible,  alcohol  offers  the  most  convenient  method  of  preser- 
vation, possessing  as  it  does  the  merits  of  simplicity  and  of  rendering 
the  tissue  receptive  to  all  forms  of  staining. 

In  the  employment  of  alcohol  for  hardening,  the  tissue  should  be 
passed  through  a  series  of  gradually-increasing  strength  ;  beginning 
with  60  per  cent,  spirit  for  2-3  days,  with  renewals  when  turbid,  the 
tissue  is  placed  successively,  at  intervals  of  3-4  days,  into  70  per 
cent.,  85  per  cent.,  and  95  per  cent,  alcohol,  finally,  after  sufficient 
hardening,  to  be  preserved  indefinitely  in  80  per  cent,  spirit. 

In  those  cases  where  bone  or  calcareous  matters  are  present,  fixation 
and  hardening  must  be  followed  by  decalcification  and  softening ; 


APPENDIX.  aYI 

this  is  most  conveniently  and  quickly  accomplished  by  placing  the 
fixed  and  partially -hardened  tissue  in  a  large  quantity  of  dilute  nitric 
acid,  varying  in  strength  from  3  to  9  per  cent.  The  fluid  should  be 
changed  daily  for  three  days,  subsequently  every  second  day.  The 
completion  of  the  decalcification  may  usually  be  determined  by  judi- 
ciously passing  a  fine  needle  into  the  tissue.  After  suspending  the 
acid  solution,  whose  too  prolonged  action  may  result  very  disastrously 
for  the  softer  parts,  the  tissue  is  thoroughly  washed  for  some  hours  in 
running  water,  and  then  placed  in  alcohols  of  gradually-increasing 
strength  to  complete  the  hardening. 

3.  Staining.  Since  the  introduction  by  Gerlach,  now  some  forty 
years  ago,  of  a  means  of  differentially  coloring  tissues,  the  list  of 
staining  methods  has  gradually  been  extended,  until  their  description 
at  the  present  time  would  cover  pages  ;  notwithstanding  the  multipli- 
cation of  formulae  and  their  claimed  advantages  for  particular  purposes, 
all  ordinary  investigations  may  be  satisfactorily  carried  on  with  the  aid 
of  a  very  limited  selection.  Among  the  important  stains,  carmine 
and  hematoxylin  stand  pre-eminent  on  account  of  their  general 
applicability  and  their  certainty.  The  relative  merits  of  carmine  and 
haematoxylin  are  well  defined  by  their  respective  advantages. 

Carmine  is,  as  usually  now  employed,  a  pure  nuclear  stain,  pos- 
sessing great  penetrating  properties,  and  hence  being  well  adapted 
for  staining  tissues  and  small  animals  in  toto, — a  matter  of  much 
importance  in  many  lines  of  work  requiring  serial  sections  ;  further, 
carmine  is  permanent,  remaining  bright  and  unfaded  after  years  of 
exposure,  does  not  overstain,  and  produces  preparations  admirably 
adapted  to  the  needs  of  the  improved  methods  of  photomicrog- 
raphy. 

Haematoxylin,  on  the  other  hand,  is  more  than  a  nuclear  stain, 
yielding,  when  successfully  employed,  beautifully  crisp  pictures  of 
cellular  structure  seldom,  if  ever,  equalled  by  carmine;  its  applicability 
in  its  usual  formulas,  however,  is  limited  to  staining  sections,  since  its 
powers  of  penetration  are  feeble.  This  latter  defect  may  be  overcome 
by  employing  the  stain  in  the  form  of  Delafield's  hematoxylin,  given 
below,  which  answers  admirably  for  bulk-staining.  The  liability  to 
fade,  the  possibility  of  overstaining,  and  the  necessity  of  using  water 
for  differentiation  are  among  the  disadvantages  of  haematoxylin  as 
usually  employed. 

The  student  is  strongly  advised  to  adopt  carmine  as  his  staple  stain, 
reserving  haematoxylin  as  a  valuable,  and  sometimes  indispensable, 
supplementary  means  of  bringing  out  parts  of  cells  not  satisfactorily 
displayed  in  carmine  preparations. 

In  the  order  of  procedure  given  above,  staining  follows  the  preser- 


4I2  APPENDIX. 

vation  of  the  tissue  and  precedes  the  embedding  and  sectioning,  this 
arrangement  being  based  on  the  supposition  that  the  tissue  is  to  be 
stained  in  bulk  and  cut  in  paraffin  :  with  this  sequence  in  view,  the 
specimen  is  removed  from  the  80  per  cent,  spirit  and  placed  directly 
in  the  staining  solution,  which,  for  all  the  ordinary  purposes  for  which 
carmine  is  employed,  is  best  made  up  as  : 

a.  Borax -Carmine  {Grenadier). 

Carmine,  best 2.5  gm. 

Borax 4.0  gm. 

Water 100  c.c. 

Alcohol  (70  per  cent. ) 100  c.c. 

The  carmine  and  borax  are  thoroughly  rubbed  up  in  a  mortar  and 
dissolved  as  far  as  possible  in  the  previously-heated  water,  the  alco- 
hol being  subsequently  added.  The  fluid  may  then  be  filtered,  but 
it  is  preferable  not  to  do  so  ;  the  solution  is  set  aside  for  at  least  two 
weeks,  and  then  carefully  decanted. 

The  exact  length  of  time  required  to  stain  sufficiently  a  block  of 
tissue  throughout  evidently  depends  upon  the  size  and  density  of 
the  specimen  ;  it  is,  however,  seldom  safe  to  trust  to  an  immersion 
of  less  than  24  hours'  duration,  and  if  the  object  be  of  large  size  and 
compact  texture,  say  a  piece  of  kidney  2  cm.  in  thickness,  it  should 
be  allowed  to  remain  in  an  ample  quantity  of  the  stain  for  at  least 
three  days.  The  vessel  containing  the  fluid  and  tissue  must  be  well 
stoppered,  a  wide-mouthed  bottle  or  tightly-covered  capsule  being 
the  most  suitable  receptacle. 

From  the  carmine  the  tissue  is  directly  transferred,  without  the 
slightest  -washing  in  water,  into  a  large  quantity  of  acid  alcohol, 
made  by  adding  strong  hydrochloric  acid  to  70  per  cent,  alcohol  in 
the  proportion  of  5  drops  of  acid  to  every  100  c.c.  of  spirit.  The 
object  of  the  acid  solution  is  to  effect  differentiation  and  fixatioyi  of 
the  color  ;  for  this  purpose  the  tissue  should  remain  at  least  24.  hours, 
and,  if  of  the  size  and  character  above  supposed,  twice  as  long — 
until  the  frequently-changed  acid  alcohol  no  longer  becomes  tinged. 
If  the  staining  has  been  successful,  the  block  of  tissue  now  appears 
of  a  brilliant  deep  uniform  red  ;  if  inspection  shows  inequality  of 
tint  or  insufficient  color  in  the  central  parts  of  the  mass,  the  staining 
will  not  be  satisfactory  and  should  be  repeated.  Failure  in  bulk 
staining  is  due  to  an  unfavorable  condition  of  the  tissue  or  to  an 
improperly-prepared  staining  fluid,  and  not  to  the  method,  which 
extended  experience  shows  is  always  capable  of  yielding  the  most 
gratifying  results,  whose  brilliancy  and  differentiation  compare  favor- 
ably with  those  of  any  carmine  staining  of  individual  sections. 

Where  it  is  preferable  to  stain  the  sections  after  cutting,  the  same 


APPENDIX. 


413 


carmine  fluid  may  be  employed,  the  sections,  either  loose  or  fixed  to 
the  slide,  being  immersed  from  15  to  20  minutes,  and  then  directly 
transferred  to  10  per  cent,  acidulated  70  per  cent,  alcohol  for  about 
5  minutes,  followed  by  thorough  washing  in  70  per  cent,  spirit. 
Where  the  tissue  is  robust,  the  acid  solution  for  differentiation  and 
fixing  may  be  made  with  water  in  place  of  the  alcohol,  water  being 
also  used  for  the  subsequent  washing  ;  it  is  an  advantage,  however, 
for  delicate  structures  to  avoid  transfers  from  alcohol  to  water,  keep- 
ing as  far  as  possible  the  sections  in  alcoholic  solutions  of  about  the 
same  strength. 

b.  Delafield's  Haematoxylin. 

(1)  Haematoxylin,  crystals 4  gm. 

(2)  Alcohol,  absolute 25  c.c. 

(3)  Ammonia-alum,  crystals 52  gm. 

(4)  Water 4°°  c.c. 

(5)  Glycerin 100  c.c. 

(6)  Methyl-alcohol 100  c.c. 

Dissolve  1  in  2,  and  3  in  4  ;  mix,  when  a  slightly-colored  fluid  is 
produced  ;  let  stand  for  4  days,  protected  from  dust,  but  with  free 
access  to  air  and  light,  at  the  end  of  which  time  the  fluid  has  turned 
to  a  deep  bluish  purple.  Filter,  and  add  5  and  6  ;  a  part  of  the  am- 
monia-alum falls  out  in  small  crystals.  After  several  hours  filter 
again,  and  then  keep  in  a  tightly-stoppered  bottle  at  least  four  or 
five  weeks  before  using. 

This  tediously-prepared  stain  possesses  the  great  advantage  of 
penetrating  and  staining  well  tissues  in  bulk,  for  many  purposes 
being  a  valuable  adjunct  to  carmine  staining.  The  strong  solution 
above  given  is  diluted  with  distilled  water,  and  the  tissue  allowed  to 
remain  until  of  a  very  dark  blue  color,  when  it  is  placed  in  distilled 
water  for  24  hours  to  effect  differentiation  and  remove  excess  of  color  ; 
it  is  then  transferred  to  70  per  cent,  alcohol  for  subsequent  treatment. 
The  action  of  the  stain  must  be  watched,  as  overstaining  may  readily 
occur  ;  should  the  coloring  be  too  intense,  this  may  be  remedied  by 
soaking  in  dilute  hydrochloric  acid,  the  action  of  the  latter  being 
arrested  at  the  proper  time  by  water,  which  at  the  same  time  restores 
the  tissue  to  its  former  blue  color,  the  acid  having  previously  turned 
it  reddish  or  brown.  It  is  very  important  to  remove  every  trace 
of  acid,  to  prevent  subsequent  fading ;  to  this  end  thorough  washing 
after  the  use  of  acid  is  imperative.  An  avoidance  of  overstaining  in 
the  first  place  is  much  more  desirable  than  any  subsequent  correction. 
In  addition  to  the  purposes  of  staining  in  bulk,  this  haematoxylin 
fluid  works  well  after  fixation  in  chromic  acid  or  Flemming's  solution, 
yielding  excellent  preparations  of  chromatin  filaments  in  such  tissues. 


414  APPENDIX. 

c.  Alum-Haematoxylin  (Bolimer). 

(i)  Hematoxylin,  ciystals 35  gm. 

(2)  Alcohol,  absolute 10  c.c. 

(3)  Potash-alum 10  gm. 

(4)  Water,  distilled 30  c.c. 

Dissolve  1  in  2  =  A  ;  dissolve  3  in  4  =  B  ;  A  is  added  to  B,  drop  by- 
drop,  and  allowed  to  stand  in  the  light  for  several  days  before  filtering. 

For  staining  sections,  dilute  with  distilled  water,  several  drops  of 
the  stain  to  a  watch-glass  of  water  usually  producing  the  requisite 
rich  bluish-purple  solution.  The  sections  remain  in  the  diluted  stain 
until  colored  dark  blue,  this  usually  requiring  5-8  minutes,  although 
the  exact  time  depends  upon  the  condition  of  the  tissue  and  the 
strength  of  the  staining  fluid  employed  ;  the  sections  are  transferred 
to  distilled  water  and  allowed  to  soak  from  5  to  10  minutes,  by  which 
time  they  have  become  a  bright  rich  blue  ;  a  too  intense  color  and  a 
light  lilac  tint  are  alike  to  be  avoided. 

The  tissue  having  been  stained  in  bulk  with  either  borax-carmine 
or  Delafield's  haematoxylin,  it  is  now  ready  for  the  important  manipu- 
lation of  embedding. 

4.  Embedding.  This  may  be  simple  or  interstitial,  the  former 
affording  a  general  support  to  the  tissue  by  grasping  its  surface  with- 
out penetrating  the  tissue,  the  latter  supporting  not  only  the  surface, 
but  also,  in  consequence  of  the  complete  infiltration  of  the  specimen, 
every  part  of  the  object.  For  the  purposes  of  hasty  examination, 
the  simple  embedding  often  answers  perfectly,  and  is  preferable  on 
account  of  economy  of  time  and  labor  ;  where,  however,  really  fine 
preparations  are  desirable,  the  additional  labor  involved  by  the  more 
elaborate  process  is  amply  repaid  by  the  character  of  the  resulting 
preparations. 

The  most  satisfactory  mass  for  simple  embedding  consists  of 
paraffin  2-3  parts  -|-  tallow  1  part ;  the  melted  mass  is  poured  around 
the  piece  of  tissue,  which  has  been  previously  fixed  in  position  by  a 
carefully-inserted  pin  within  a  paper  mould.  The  mass  should  cool 
slowly,  and  the  sections  should  be  cut  with  both  knife  and  block 
flooded  with  strong  spirit. 

Interstitial  Embedding,  by  which  every  portion  of  the  entire 
tissue-mass  is  held  together  and  sustained,  each  isolated  fragment 
being  retained  in  its  relative  position  and  preserved  in  the  mounted 
preparation,  has  given  the  histologist  of  to-day  a  command  of  his 
tissues  incomparably  superior  to  anything  that  his  predecessors  pos- 
sessed, and  enables  him  to  secure  complete  series  of  objects  whose 
minuteness  and  frailty  precluded  perfect  preparations  by  the  older 
methods. 


APPENDIX.  4Ic 

The  important  processes  of  interstitial  embedding  are  two,  paraffin 
and  celloidin  being  the  substances  respectively  used  to  infiltrate  the 
tissue  ;  of  these  the  paraffin  method  must  be  regarded  as  the  most 
perfect,  and,  with  few  exceptions,  to  be  preferred  whenever  thin  per- 
fect sections  are  of  importance,  especially  where  preservation  of 
sequence  is  desirable. 

Paraffin  Method  (Klebs). 

The  essential  point  of  this  process  is  thorough  and  complete  im- 
pregnation of  the  tissue  with  the  embedding  mass  ;  it  is  conse- 
quently necessary  to  saturate  the  tissue  with  some  fluid  with  which 
the  paraffin  is  perfectly  miscible,  the  fluids  usually  employed  to  this 
end  being  chloroform  or  turpentine  oil  ;  in  order,  however,  to  insure 
the  free  entrance  of  these  fluids  within  the  tissue,  it  is  first  necessary 
to  free  the  tissue  of  all  traces  of  water  still  contained  in  the  alcohols 
of  70  or  80  per  cent.  It  is,  therefore,  necessary  to  place  the  tissue 
from  the  usual  80  per  cent,  preserving  spirit  as  follows  :  * 

a.  Into  95 per  cent,  alcohol  from  12  to  24  hours. 

b.  Into  absolute  alcohol  from  24  to  48  hours,  until  complete  dehy- 
dration has  been  secured  ;  this  step  is  of  the  utmost  importance  for 
the  success  of  all  the  subsequent  manipulations,  since  if  dehydration 
be  imperfect  infiltration  will  be  unsuccessful,  and  the  tissue  will  cut 
badly. 

c.  Into  pure  chloroform  from  6  to  8  hours,  or  until  the  chloroform 
has  replaced  the  absolute  alcohol  ;  an  indication  of  the  completion 
of  this  interchange  is  furnished  by  the  position  of  the  tissue,  since  as 
soon  as  the  tissue  continues  to  lie  beneath  the  surface  of  the  chloro- 
form, or  sinks  towards  the  bottom  of  the  bottle,  it  may  be  concluded 
that  the  alcohol  has  been  completely  replaced  by  the  chloroform. 

d.  Into  a  saturated  solution  of  paraffin  in  chloroform  from  2  to  3 
hours  ;  the  solution  may  be  kept  slightly  warmed. 

e.  Into  pure  melted  paraffin,  which  has  a  melting-point  of  about 
500  C. ;  the  paraffin  is  best  contained  in  a  small  open  porcelain  cap- 
sule placed  within  a  water-oven  so  regulated  as  to  maintain  a  constant 
temperature  of  about  500  C. ;  while  undesirable,  congealment  of  the 
surface  of  the  paraffin  due  to  reduction  in  temperature  is  no  great 
misfortune,  the  retarded  evaporation  of  the  chloroform  being  the 
principal  evil  ;  a  rise  of  the  temperature  to  which  the  tissue  is  sub- 
jected to  a  point  beyond  550  C,  on  the  contrary,  is  usually  disastrous, 
the  tissue  being  shrunken  and  distorted  to  a  degree  which  renders  it 
valueless.     It  is,   therefore,  desirable  to  keep  the  paraffin  and  the 

*  In  the  appended  data  it  is  still  assumed  that  the  tissue  being  treated  is  of  the 
consistence  and  volume  represented  by  a  piece  of  kidney  2  cubic  cm.  in  size. 


4I5  APPENDIX. 

tissues  at  a  temperature  whose  variations  are  included  within  the 
limits  of  500  to  520  C. 

The  tissue  must  remain  in  the  melted  paraffin  until  every  portion 
of  it  has  been  completely  filled  with  the  embedding  mass  and  all  the 
chloroform  has  been  driven  off.  This  latter  point  is  a  matter  of 
importance  in  insuring  the  proper  consistence  of  the  paraffin  for  satis- 
factory cutting,  since  when  the  paraffin  contains  traces  of  chloroform 
it  is  too  soft  and  friable  to  yield  the  best  results.  In  order  to  deter- 
mine whether  all  the  chloroform  has  been  driven  off,  a  clean  thin  iron 
rod  is  heated  and  plunged  into  the  melted  paraffin,  care  being  taken 
that  the  rod  is  not  too  hot  when  immersed,  lest  the  tissue  be  over- 
heated. So  long  as  traces  of  chloroform  are  present,  bubbles  follow 
the  introduction  of  the  heated  rod  ;  when  bubbles  no  longer  appear, 
all  the  chloroform  has  been  driven  off. 

After  the  complete  dissipation  of  the  chloroform,  the  tissue  is  trans- 
ferred for  a  few  minutes  to  a  second  capsule  containing  fresh,  unused, 
melted  paraffin  of  such  consistence  as  is  best  adapted  to  sectioning 
under  the  conditions  of  season  and  particular  object  in  view  ;  the 
quantity  of  melted  paraffin  should  amply  suffice  to  fill  the  mould 
which  is  to  be  employed  in  the  next  manipulation. 

f.  Embedding  the  Tissue.  For  this  purpose  some  form  of  mould 
must  be  devised,  which  may  be  the  simple  paper  box,  made  by  fold- 
ing over  a  block  the  sides  and  ends  of  a  piece  of  sized  paper  some 
4  cm.  wide  by  8  cm.  long  ;  more  convenient  are  the  adjustable  metallic 
embedding  frames  furnished  by  dealers,  those  made  by  Jung,  of 
Heidelberg,  and  sold  by  various  firms  in  this  country,  being  par- 
ticularly serviceable.  When  the  paper  box  is  used,  it  should  be  fixed 
to  a  loaded  cork  before  the  paraffin  is  poured  into  it;  when  the  metallic 
frame,  it  must  closely  rest  upon  a  piece  of  polished  glass.  In  either 
case,  the  mould  is  placed  in  a  broad  dish,  whose  depth  somewhat 
exceeds  the  height  of  the  sides  of  the  mould  when  resting  in  position 
for  use. 

The  mould  and  dish  being  ready,  the  capsule  containing  the  fresh 
paraffin  and  specimen  is  removed  from  the  oven,  and  the  paraffin 
poured  at  once  into  the  mould,  which  should  be  completely  filled  ; 
after  this  has  been  done,  the  tissue  is  seized  lightly  with  the  slightly- 
warmed  forceps,  and  rapidly  transferred  to  the  mould  ;  a  warm  needle 
should  be  at  hand  with  which  to  arrange  the  tissue,  so  that  the  pro- 
posed plane  of  section  shall  lie  parallel  to  one  of  the  smaller  ends  of 
the  mould,  while  its  principal  axis  corresponds  with  the  bottom  upon 
which  the  tissue  rests. 

As  soon  as  the  specimen  is  properly  placed — and  this  is  often  a 
matter  of  great  consequence — steps  should  be  taken  to  harden  the 
paraffin  as  rapidly  as  possible.     To  this  end,  the  dish  supporting  the 


APPENDIX.  4I- 

mould  and  specimen  should  be  filled  with  cold  water  until  the  latter 
is  just  on  the  point  of  overflowing  the  sides  of  the  mould,  great  care 
being  taken  that  this  does  not  happen  before  the  surface  of  the  enclosed 
lake  of  melted  paraffin  has  congealed,  otherwise  the  paraffin  becomes 
partially  displaced  by  the  water,  which  will  be  found  later  within  large 
cavities  in  the  block. 

As  soon  as  the  film  on  the,surface  has  completely  formed,  the  water 
is  allowed  to  cover  the  mould  entirely  ;  the  dish  may  then  be  placed 
under  the  tap,  and  a  gentle  stream  of  water  aid  in  cooling  off  the 
mass.  No  attempt  should  be  made  to  remove  the  block  from  the 
mould  until  the  entire  mass  has  become  thoroughly  hardened ;  when 
this  has  occurred,  and  the  embedded  object  with  its  surrounding  mass 
has  been  freed,  the  paraffin  should  appear  almost  transparent  and 
of  a  bluish  tint,  and  not  milk-white,  as  is  usually  the  case  when  the 
paraffin  is  impure  or  when  the  block  has  been  cooled  slowly.  After 
trimming  off  the  superfluous  embedding  material  and  exposing  the 
surface  to  be  sectioned,  the  tissue  is  ready  for  cutting.  Objects  may 
be  preserved  within  the  paraffin  indefinitely,  the  method  affording  an 
admirable  and  convenient  means  of  keeping  tissues  for  any  length  of 
time  and  always  ready  for  immediate  sectioning  and  mounting. 

Celloidin  Method  {Duval- Schiefferdecker). 

This  method  has  but  one  point  in  common  with  the  paraffin  process 
— the  tissue  is  infiltrated  with  the  embedding  mass  ;  while  paraffin  is 
cut  dry,  celloidin  must  be  cut  under  or  flooded  with  spirit.  Celloidin 
is  particularly  adapted  for  certain  lines  of  work  in  the  central  nervous 
system  and  the  special  senses,  and  possesses  the  advantages  over 
paraffin  of  requiring  less  attention  and  no  heat  for  its  successful  manip- 
ulation. The  retention  of  the  supporting  mass,  the  rather  thicker 
sections,  and  the  impossibility  of  cutting  ribbon-series,  on  the  other 
hand,  are  points  of  unfavorable  comparison  with  the  paraffin  method. 

The  celloidin  should  be  prepared  as  two  solutions,  a  thin  and  a 
thick :  the  celloidin — either  as  chips  or  in  cake — is  dissolved  in  equal 
parts  of  absolute  alcohol  and  ether,  about  5  grammes,  in  small  pieces, 
being  placed  in  100  c.c.  of  the  mixed  solvent  ;  the  resulting  solution 
will  be  very  thin,  and  maybe  labelled  "A"  ;  a  second  solution  should 
be  made  containing  enough  celloidin  to  secure  the  consistence  of  a 
thick  syrup;  this  is  "B."  The  celloidin  does  not  dissolve  with 
great  readiness,  days  often  being  required  for  the  preparation  of  the 
solutions  ;  these  should  be  very  carefully  gziarded  against  evapora- 
tion, and  a  small  quantity  of  the  absolute  alcohol  and  ether  added 
from  time  to  time  to  maintain  the  proper  degree  of  fluidity. 

The  tissue,  previously  thoroughly  dehydrated  by  absolute  alcohol, 
is  soaked  in  a  mixture  of  equal  parts  of  absolute  alcohol  and  ether 

27 


4i8 


APPENDIX. 


from  24  to  48  hours  and  then  transferred  to  the  "A"  or  thin  celloidin 
solution,  in  which  it  remains  for  several  days,  until  entirely  permeated 
with  the  mass  ;  the  tissue  is  then  placed  in  the  thick  "  B"  solution, 
where  it  stays  until  the  thinner  fluid  has  been  replaced  by  the  thicker. 
Meanwhile,  corks  of  suitable  size  should  be  soaking  in  the  mixture 
of  equal  parts  of  absolute  alcohol  and  ether  ;  one  of  these  is  selected, 
its  end  slightly  roughened,  and  finally  moistened  with  a  few  drops 
of  the  mixture  just  before  the  tissue  with  an  envelope  of  the  thick 
celloidin  solution  is  placed  in  position  for  cutting  on  the  cork,  care 
being  taken  that  the  stratum  (1-2  mm.)  of  celloidin  lies  between  the 
tissue  and  the  cork.  After  a  few  moments  a  fresh  layer  of  celloidin 
is  added,  and  this  process  is  repeated  until  the  tissue  is  completely 
surrounded  with  a  stratum  of  the  embedding  mass  ;  or  the  tissue 
may  be  completely  embedded,  after  being  attached  to  the  surface  of 
the  cork,  by  fastening  a  piece  of  writing-paper  to  the  sides  of  the 
cork  and  at  once  filling  the  resulting  mould  with  the  fluid  celloidin. 
The  mass  of  fresh  celloidin  should  remain  undisturbed  until  the  sur- 
face has  hardened  sufficiently  to  prevent  all  possibility  of  shifting, 
when  the  cork  with  the  tissue  is  transferred  to  a  vessel  containing 
yS  per  cent,  spirit  to  harden,  where  it  remains,  completely  immersed, 
from  1  to  3  days  or  longer  ;  at  the  expiration  of  this  time  the  block 
has  gained  a  consistence  suitable  for  sectioning.  The  cutting  can  be 
done  either  free-hand  or  by  the  microtome,  but  it  must  be  done 
while  both  knife  and  tissue  are  flooded  with  70  per  cent,  alcohol. 
The  sections  are  transferred  to  70  per  cent,  spirit  for  subsequent 
treatment  ;  if  already  stained,  they  are  passed  into  95  per  cent,  for 
dehydration,  cleared  in  xylol,  bergamot  oil,  or  cedar  oil  (but  not  in 
clove  oil,  as  this  dissolves  the  celloidin),  and  mounted  in  balsam  ;  if, 
on  the  other  hand,  the  tissue  has  not  been  stained  in  bulk,  the  sec- 
tions are  treated  with  the  selected  stain,  alcoholic  or  aqueous,  and 
subsequently  dehydrated,  cleared,  and  mounted. 

5.  Sectioning.  While  for  the  purposes  of  immediate  examination 
or  of  temporary  preparation  free-hand  sections  often  suffice,  yet  no 
one  seriously  undertaking  histological  investigation  can  afford  to 
ignore  the  advantages  possessed  by  the  approved  microtomes  of  the 
present  day,  without  which  accurate  work  is  impossible.  After  an 
extended  experience  with  many  forms  of  these  instruments,  the 
writer  unhesitatingly  recommends  the  sliding  microtome,  as  made  by 
Schanze,  of  Leipsic,  as  the  best  all-round  instrument  to  be  had,  the 
medium-sized  "model  B"  of  this  maker  supplying  an  ideal  tool 
capable  of  executing  all  forms  of  cutting  in  the  most  satisfactory  and 
convenient  manner.  The  little  sliding  student's  microtome  made  by 
the  Bausch  and  Lomb  Optical  Company  answers  as  a  very  satisfactory 


APPENDIX.  41g 

substitute  for  the  more  convenient  and  accurate  foreign  instrument. 
A  word  of  caution  may  be  added  against  regarding  all  forms  of  sliding 
microtomes  as  equally  efficient,  since  the  satisfactory  working  of  such 
tools  is  largely  dependent  upon  details  of  their  construction  and 
workmanship.  While  theoretically  more  accurate,  the  beautifully- 
made  Thoma  microtomes  with  the  ' '  Naples' '  holder  are  much  less 
convenient  than  the  Schanze  instruments,  and  are  less  desirable  than 
the  latter  for  general  use.  Where  much  ribbon-cutting  is  carried  on, 
the  Minot  automatic  microtome  will  be  found  a  most  valuable  time- 
and  labor-saving  device  ;  equally  perfect  ribbon-series  can  be  pro- 
duced on  the  Schanze,  but  with  much  less  rapidity. 

Assuming  that  some  satisfactory  form  of  sliding  microtome  is  at 
command,  and  that  the  tissue  has  been  embedded  interstitially  in 
paraffin,  the  method  employed  in  cutting  will  depend  on  whether 
isolated  sections  or  a  series  are  desired.  For  very  many  purposes 
the  separate  sections  are  all  that  is  needed,  their  relative  position  and 
sequence  being  preserved  by  systematic  arrangement  as  they  are 
cut.  In  making  such  individual  sections,  the  knife  should  be  placed 
obliquely  to  the  tissue,  the  exact  angle  being  such  that  the  entire 
length  of  the  plate  is  successively  brought  into  use.  It  will  be  found 
necessary  to  adopt  some  means  of  preventing  the  rolling  up  of  the 
sections  as  they  are  cut,  this  tendency  being  especially  pronounced 
with  the  harder  grades  of  paraffin  ;  after  this  has  occurred,  the  sec- 
tions are  usually  useless.  The  simplest  and  most  effective  means  of 
overcoming  this  difficulty  is  to  hold  a  small  red  sable  brush  over  the 
edge  of  the  knife,  and,  as  the  latter  enters  the  block,  lightly  hold  the 
section  as  it  is  being  cut  from  curling  over  and  rolling  up  ;  the  manip- 
ulation requires  some  little  dexterity,  but  when  once  acquired  sup- 
plies a  simple  "section  smoother"  equally  as  efficient  as  any  of  the 
more  elaborate  mechanical  devices.  In  cutting  paraffin  sections,  no 
fluid  is  needed,  both  block  a?id  knife  being  kept  perfectly  dry.  The 
knife  should  be  wiped  occasionally  with  a  clean  cloth,  to  remove  any 
particles  of  the  embedding  mass  that  may  adhere  ;  especial  attention 
must  be  given  to  the  edge  and  under  surface  of  the  blade,  as  some- 
times a  minute  adherent  fragment  will  cause  cracks  across  the  entire 
surface  of  the  subsequent  sections.  The  forceps  or  a  brush  serve  to 
transfer  the  sections  from  the  knife-blade  to  the  adjacent  tray  lined 
with  perfectly  clean  paper,  upon  which  the  sections  may  remain  for  a 
long  time  if  properly  guarded  against  high  temperature  and  dust. 
The  average  thickness  of  satisfactory  paraffin  sections  is  about  .01 
mm. ;  large  sections  are  usually  somewhat  thicker,  small  delicate 
objects,  as  embryos,  readily  yielding  sections  not  much  over  half  as 
thick  (.005  to  .008  mm.)  ;  it  should  not  be  forgotten  that  a  keen  blade 
and  proper  paraffin  are  essential  to  first-class  results. 


420 


APPENDIX. 


In  cutting  celloidin  or  other  specimens  requiring  to  be  flooded 
while  sectioned,  the  knife  is  likewise  placed  obliquely  ;  the  sections 
are  removed  with  a  soft  wet  brush  and  transferred  to  alcohol ;  rolling 
up  of  such  sections  does  not  occur. 

Cutting  ribbon-series  is  a  modification  of  the  usual  procedure, 
and  possesses  great  advantages  where  the  possession  of  a  complete 
series  of  sections  arranged  in  their  natural  sequence  is  important ;  not 
only  for  embryological  studies,  where  it  has  become  a  necessity,  but  for 
many  other  purposes,  ribbon-cutting  is  to  be  preferred.  The  success 
of  the  manipulation  depends  largely  upon  the  proper  consistence  of 
the  paraffin,  since  the  latter  must  be  of  just  such  hardness  that  while 
firm  enough  to  enable  the  sections  as  cut  to  push  before  them  those 
already  in  the  chain,  it  must  be  sufficiently  soft  to  enable  the  op- 
posed edges  of  the  sections  to  adhere  together,  and  thus  form  the 
"  ribbon."  Preparatory  to  cutting  the  paraffin  block  is  trimmed  as 
accurately  as  possible  into  rectangular  form,  and  so  clamped  in  the 
microtome  that  the  longer  sides  of  the  rectangle  are  exactly  parallel  to 
the  edge  of  the  transversely -set  knife,  the  latter  being  placed  at  right 
angles  to  its  slide-ways. 

When  the  first  section  is  cut  it  is  not  removed,  but  allowed  to  lie 
upon  the  blade  ;  the  knife  being  returned  to  its  first  position,  the 
tissue  is  raised  the  proper  distance  (generally  .01  mm.),  and  a  second 
section  is  made,  which,  if  the  paraffin  is  of  the  proper  character, 
will  adhere  to  the  first,  while  the  latter  is  pushed  ahead  for  a  distance 
equal  to  the  second  section  ;  in  this  manner  each  section  in  turn  drives 
those  previously  cut  before  it,  all  adhering  by  their  opposed  edges 
and  forming  a  ribbon  whose  length  is  often  limited  only  by  the  wishes 
and  the  convenience  of  the  worker.  Care  must  be  taken  to  keep  the 
cutting  edge,  especially  its  under  surface,  free  from  particles  of  par- 
affin, since  the  presence  of  these  will  lead  to  furrows  and  cracks  in 
the  sections.  The  sides  of  the  block  corresponding  to  the  knife- 
edge  must  also  be  kept  exactly  parallel,  otherwise  the  ribbon  will  be 
curved  instead  of  straight.  In  case  the  paraffin  in  which  the  tissue 
lies  is  too  hard,  the  sections  breaking  apart  instead  of  adhering,  ele- 
vating the  temperature  of  the  workroom  or  judiciously  holding  the 
block  in  the  vicinity  of  a  flame  for  a  short  time  will  usually  afford 
relief;  or  the  entire  block  may  be  coated  with  softer  paraffin,  which 
is  subsequently  trimmed  off  from  all  but  the  two  adhering  sides.  As 
the  ribbons  are  completed  they  are  placed  in  covered  trays  upon 
clean  sized  paper,  protected  from  dust  and  high  temperature. 

6.  Fixing  sections  to  the  slide  constitutes  the  next  step  after 
cutting  when  tissues  have  been  embedded  in  paraffin,  whether  pre- 


APPENDIX. 


421 


'viously  stained  or  not  ;  the  object  of  the  manipulation  is  to  replace 
the  support  afforded  by  the  paraffin  by  attaching  the  sections  to  the 
slide  before  removing  the  embedding  substance.  In  this  point  paraf- 
fin is  much  more  accurately  and  conveniently  worked  than  celloidin, 
since  the  latter  is  removed  from  the  sections  with  much  less  facility 
than  paraffin.  The  ideal  fixing  solution  is  yet  to  be  devised,  those 
at  present  employed  being  all  defective  in  some  particular.  The 
desiderata  are  secure  attachment  of  the  sections  to  the  slide  in  all 
solutions  necessary  for  the  various  manipulations  of  staining  and 
mounting,  and  complete  expa?ision  of  the  sections  before  their  final 
adhesion  to  the  slide  :  this  latter  consideration  is  of  great  importance 
in  large  sections  or  in  mounting  ribbon-series,  since  it  is  practically 
impossible  to  cut  these  without  some  slight  compression  or  wrinkling; 
if  mounted  without  being  perfectly  expanded,  the  preparations  are 
marred  by  distorting  folds,  which  in  lines  of  accurate  work,  where 
reconstructions  are  sometimes  necessary,  are  serious  defects.  The 
most  satisfactory  fixing  solutions  are  the  gum  arabic  (Flogel- 
Schultze)  and  collodion-clove  oil  (Sch'dllibauni)  mixtures. 

The  gum-arabic  method  is  carried  out  as  follows  :  of  a  saturated 
aqueous  solution  of  best  gum  arabic  (a  crystal  of  thymol  being  added 
to  prevent  the  growth  of  fungi)  about  12  drops  are  added  to  30  c.c. 
of  distilled  water  and  thoroughly  shaken.  The  slide  is  flooded  with 
the  solution,  care  being  taken  that  the  fluid  does  not  run  over  the 
edges,  and  the  sections  are  floated  on  the  liqicid,  every  part  of  the 
sections  being  separated  from  the  slide  by  the  stratum  of  solution  : 
when  all  the  sections  are  arranged,  the  slide  is  transferred  to  a  warm- 
ing-plate and  very  gently  heated  to  a  temperature  never  as  high  as 
the  melting-point  of  the  paraffin,  the  object  being  to  enable  the 
sections  to  expand  while  swimming  on  the  gum  sohdion  ;  this  they 
do  in  a  most  satisfactory  manner  within  a  few  minutes,  the  sections 
spreading  out  perfectly  flat  even  when  previously  wrinkled. 

After  expansion  the  excess  of  the  fluid  is  drained  off,  and,  if  neces- 
sary, the  sections  finally  rearranged  ;  the  slides  are  then  placed  in  a 
suitable  place  to  dry,  where  evaporation  is  favored  but  protection 
from  dust  is  afforded.  It  usually  is  best  to  allow  the  sections  to  lie 
overnight  to  insure  complete  drying,  as  if  water  be  still  present  the 
sections  will  not  properly  clear  up. 

The  disadvantages  of  the  method  are  the  long  time  required  to 
insure  complete  evaporation  of  the  fluid  and  the  inability  of  sections 
so  fastened  to  withstand  watery  solutions,  which  dissolve  the  gum 
and  loosen  the  sections.  These  objections,  however,  are  more  theo- 
retical than  real,  and  are  more  than  compensated  by  the  superior 
preparations  secured  by  this  method  ;  in  the  exceptional  cases  where 
it  is  necessary  to  apply  aqueous  solutions,  advantage  may  be  taken 


422 


APPENDIX. 


of  the  modification  introduced  by  Gray,  who  uses  a  weak  gelatin 
solution  in  place  of  the  gum  arabic,  and,  after  the  sections  have  ex- 
panded and  are  fastened  in  their  proper  positions,  soaks  the  slide  in 
a  very  weak  solution  of  potassium  bichromate,  which,  in  the  presence 
of  light,  renders  the  gelatin  film  insoluble  in  water,  and  hence  capable 
of  resisting  aqueous  stains.  To  those  desiring  accurate  preparations, 
these  methods  are  strongly  recommended  as  preferable  to  the  more 
rapidly  applied  and  generally  used 

Collodion  and  Clove-Oil  Mixture.  This  is  made  by  adding  i 
part  of  collodion  to  3  parts  of  clove  oil ;  the  mixture  should  be  made 
up  in  small  quantities,  as  it  becomes  less  reliable  with  age.  The  slide 
is  lightly  painted  over  with  the  mixture  and  the  paraffin  sections 
placed  in  position  ;  the  sections  cannot  be  moved  after  touching  the 
mixture,  hence  care  must  be  exercised  in  their  placing.  When  the 
slide  is  full,  it  is  gently  wanned  iintil  the  fumes  of  the  clove  oil  ap- 
pear ;  meanwhile  the  paraffin  melts  and  the  section  sinks  down  into 
the  film  of  the  mixture,  from  which  the  clove  oil  is  driven  off,  leaving 
the  tissue  attached  to  the  slide  by  the  film  of  collodion  alone  ;  this 
union  is  not  attacked  by  any  of  the  aqueous,  alcoholic,  or  other  solu- 
tions ordinarily  used.  The  ability  of  resisting  many  fluids,  together 
with  its  simplicity  and  rapidity,  has  long  rendered  the  method  a 
favorite,  and,  for  very  many  cases,  deservedly  so,  due  care  being 
exercised  in  heating  the  slide  to  avoid  injury  to  the  tissue.  In  spite, 
however,  of  these  considerable  advantages,  the  inability  of  securing 
perfect  extension  of  the  sections  is  a  shortcoming  which  for  accurate 
investigations  is  fatal  ;  when,  therefore,  accurate  preparations  are  de- 
sired, it  should  be  discarded  for  the  gum  or  the  gelatin  method.  The 
sections  being  securely  fixed  to  the  slide  by  one  of  the  foregoing  meth- 
ods, the  paraffin  on  the  slide  must  be  removed,  as  preliminary  to — 

7.  Mounting  the  sections  for  preservation.  The  paraffin  is  best 
removed  by  immersing  the  slide  in  benzole  or  xylol  for  a  few  mo- 
ments and  then  transferring  to  turpentine  for  a  short  time.  The 
sections  having  cleared  up  in  these  fluids  are  ready  for  the  applica- 
tion of  the  mounting  medium,  the  balsam.  The  slide  is  removed 
from  the  turpentine,  drained,  and  hastily  wiped  on  the  back  and 
edges,  care  being  taken  not  to  touch  the  sections  ;  a  drop  of  pure 
balsam  is  then  placed  on  the  centre  of  the  slide  and  the  latter  held 
for  a  few  moments  over  a  spirit  flame  to  liquefy  more  thoroughly  the 
balsam,  when  the  cleaned  cover-glass,  previously  caught  by  the  for- 
ceps and  passed  for  a  moment  over  the  spirit  flame,  is  lowered  into 
position  ;  this  manipulation  should  be  executed  with  steadiness  and 
evenness,  avoiding  as  far  as  possible  the  imprisonment  of  air-bubbles. 
Should  these,  however,  appear  after  the  cover  is  in  place,  they  need 


APPENDIX.  423 

cause  no  concern,  as  they  usually  spontaneously  disappear  during  the 
next  twelve  hours  unless  imprisoned  within  some  enclosed  recess 
of  the  tissue  ;  rough  treatment,  by  strongly  and  repeatedly  pressing 
on  the  cover-glass  in  the  attempt  to  displace  air-bubbles,  is  disastrous 
to  thin  sections,  and  should  never  be  practised  ;  ge?itle  pressure, 
however,  should  be  made  after  the  cover  is  down,  to  press  out  super- 
fluous balsam,  but  this  must  be  done  with  care  and  judgment.  The 
balsam  should  entirely  fill  up  the  space  beneath  the  cover  and  form 
a  slight  border  outside  ;  this  edging  of  balsam  is  useful,  as  it  dries 
much  sooner  than  the  medium  beneath  the  cover  and  adds  very 
materially  to  the  strength  of  the  preparation. 

The  freshly-mounted  slides  should  be  placed  in  the  horizontal 
position  and  allowed  to  dry  some  days  before  being  much  handled, 
although  if  necessary  a  preliminary  study  of  them  can  be  made  at 
once.  No  attempt  should  be  made  to  clean  them  until  the  balsam 
has  well  hardened  and  all  danger  of  moving  the  cover-glass  disap- 
peared ;  the  excess  of  the  mounting  medium  is  then  removed  with 
a  sharp  knife  and  the  slide  finally  cleaned  by  a  cloth  moistened  with 
benzole. 

8.  Finishing,  labelling,  and  storing  the  slides  depend  largely 
upon  the  individual  taste  and  wishes  of  the  worker  ;  while  the  earnest 
investigator  has  little  time  for  useless  ornamentation,  the  small  amount 
of  labor  involved  in  having  slides  clean,  neat,  and  properly  labelled 
is  well  compensated  by  the  convenience  and  satisfaction  of  handling 
such  preparations.  Labels  should  always  be  attached  as  soon  as 
practicable,  and  should  indicate  all  data  likely  to  be  of  interest ; 
when  labels  are  placed  on  both  ends  of  the  slide,  one  should  be  re- 
served for  noting  points  of  especial  interest  shown  by  the  preparation. 
In  preparing  slides  on  which  an  entire  series  is  mounted,  marking 
each  with  a  diamond  saves  much  vexatious  delay,  which  otherwise  is 
often  experienced  in  determining  the  proper  sequence.  Finished 
preparations  are  best  preserved  in  some  form  of  cabinet  or  case,  the 
exact  character  of  which  is  of  little  consequence  so  long  as  the  slides 
are  protected  from  dust  and  light  and  lie  flat ;  cabinets  with  well-made 
drawers  are  attractive  and  convenient,  but  usually  expensive. 

In  recapitulation  of  the  foregoing  manipulations,  already  considered 
in  detail,  the  steps  necessary  to  convert  the  fresh  tissue  into  the 
finished  preparation  may  be  presented  as 

AN    OUTLINE    OF    THE   STANDARD    METHOD. 

i.  Fixation  of  fresh  tissue  in  large  quantity  of  Miiller's  fluid  ; 
renewal  when  turbid  ;  tissue  remains  2-3  weeks. 

2.   Thorough  washing  in  running  water — 2-5  hours. 


424  APPENDIX. 

3.  Transfer  to  70  per  cent,  alcohol ;  keep  in  dark  ;  change  alcohol 
whenever  it  becomes  deeply  tinged,  until  it  remains  colorless. 

4.  Stain  in  excess  of  borax-carmine — 24-48  hours. 

5.  Transfer  directly,  without  washing,  from  stain  to  acid  alcohol 
— 24-48  hours. 

6.  Wash  well  in  70  per  cent,  alcohol,  several  times  renewed — 24 
hours. 

7.  Transfer  to  80  per  cent,  alcohol — 24  hours. 

8.  Transfer  to  95  per  cent,  alcohol — 24-48  hours. 

9.  Dehydrate  in  absolute  alcohol — 24-48  hours. 

10.  Transfer  to  pure  chloroform  until  tissue  sinks — 6-8  hours. 

11.  Transfer  to  saturated  solution  of  paraffin  in  chloroform — 6 
hours. 

12.  Transfer  to  pure  melted  paraffin,  kept  at  constant  temperature 
of  about  500  C,  until  all  chloroform  is  driven  off— 6-8  hours. 

13.  Transfer  to  fresh  melted  pure  paraffin  of  consistence  for  em- 
bedding— 10-15  minutes. 

14.  Embed  tissue  in  mould  ;  cool  rapidly. 

15.  Section  in  microtome,  first  suitably  trimming  block  for  cutting. 

16.  Fix  sections  to  slides  by  gum  or  collodion-clove  oil. 

17.  Remove  paraffin  by  benzole,  succeeded  by  turpentine. 

18.  Drain  off  excess  of  turpentine,  apply  balsam,  and  cover. 

19.  Place  freshly-mounted  slides  in  horizontal  position. 

20.  Clean  up  and  permanently  label  when  thoroughly -dry  ;  store 
in  suitable  cabinet. 

While  the  duration  of  the  several  manipulations  as  indicated  in 
the  above  summary  represents  the  time  usually  required  by  ordinary 
objects,  yet  the  individual  character  of  the  tissue  must  be  considered 
in  each  case,  as  density  exerts  much  influence  on  the  rapidity  with 
which  the  fluids  penetrate. 

When  it  is  desirable  to  stain  the  tissue  after  sections  have  been 
cut,  the  above  manipulations  must  be  modified  ;  steps  4,  5,  and  6  in 
such  case  are  omitted  and  the  tissue  is  at  once  dehydrated.  Re- 
moval of  the  paraffin  from  the  fixed  sections  on  the  slides  (17)  by 
benzole  is  at  once  succeeded  by  the  following  manipulations  : 

a.  Transfer  to  95  per  cent,  to  remove  the  benzole — 5-10  minutes. 

b.  Transfer  to  clean  95  per  cent,  alcohol  to  insure  complete  absence 
of  benzole — 5  minutes. 

c.  Transfer  to  80  per  cent,  alcohol — 5  minutes. 

d.  Transfer  to  70  per  cent,  alcohol — 5  minutes. 

e.  Stain  in  borax-carmine  solution — 10-15  minutes. 

f.  Differentiate  in  acid  alcohol  (10  per  cent.) — 6-10  minutes. 

g.  Wash  in  70  per  cent,  alcohol,  renewed — 10-15  minutes. 
h.  Transfer  to  80  per  cent,  alcohol — 15  minutes. 


APPENDIX.  ,0- 

i.  Transfer  to  95  per  cent,  alcohol — 15  minutes. 

j.  Dehydrate  thoroughly  in  absolute  alcohol — 15  minutes. 

k.  Clear  sections  in  oil  of  turpentine — 5  minutes. 

1.  Mount  in  balsam  as  indicated  above  in  18. 

When  hematoxylin  is  used  as  the  stain,  the  steps  e,  f,  and  g  are 
omitted  and  replaced  by — 

ee.  Transfer  to  distilled  water — 5  minutes. 

ff.  Stain  in  properly-diluted  hematoxylin  fluid  until  sufficiently 
dark — 8-10  minutes. 

gg.  Wash  well  in  distilled  water  to  remove  excess  of  stain  and  to 
differentiate — 10  minutes  ;  then  dehydrate  by  the  ascending  series  of 
alcohols  included  by  h  to  j  as  above. 

The  foregoing  methods  are  those  to  be  employed  as  the  standard 
processes,  since  for  the  great  majority  of  specimens  they  yield  results 
perfectly  satisfactory  and  trustworthy  ;  sometimes,  however,  special 
lines  of  investigation  demand  other  treatment  in  order  to  bring  out 
particular  features.  Several  of  those  most  important  for  the  study 
of  the  nervous  system  are  here  given. 

Weigert's  haematoxylin  method  is  of  great  value  in  exhibiting 
the  presence  and  course  of  medullated  nerve-fibres  on  account  of  the 
peculiar  staining  of  the  medullary  substance  ;  the  method  takes  ad- 
vantage of  the  tenacity  with  which  this  part  of  the  nerve-fibre  retains 
the  color,  appearing  slate-blue  or  black,  while  the  other  parts  of  the 
nervous  tissues  become  pale  ;  the  tissue  is  first  overstained  and  then 
decolored. 

The  fresh  spinal  cord  or  the  brain  is  hardened  in  a  large  excess  of 
potassium  bichromate  (5  per  cent,  solution),  repeatedly  renewed,  for 
several  weeks,  and  then  directly  transferred  to  80  per  cent,  alcohol, 
kept  in  the  dark,  and  frequently  changed  until  the  fluid  is  no  longer 
discolored  ;  as  the  tissue  is  usually  cut  in  celloidin,  the  next  step  is 
the  dehydration  by  95  per  cent,  and  absolute  alcohol,  followed  by  the 
usual  process  of  the  celloidin  embedding.  After  this  has  been  ac- 
complished, and  the  tissue  is  on  the  cork  ready  for  cutting,  the  entire 
block  is  transferred  to — 

a.  Saturated  solution  neutral  cupric  acetate     1  part ; 

Solution  of  Rochelle  salt  (10  per  cent.) 1  part, 

for  24  hours  in  oven  at  400  C. 

b.  Transfer  to — 

Saturated  solution  neutral  cupric  acetate 1  part ; 

Distilled  water 1  part, 

for  24  hours. 


426  APPENDIX. 

c.  Transfer  to  80  per  cent,  alcohol — %—i  hour. 

d.  Cut  sections  ;  knife  and  tissue  wet  with  80  per  cent,  alcohol. 
The  later  method  of  Weigert  directs  the  preparation  of  the  fol- 
lowing staining  solutions  : 

A     i  Lithium  carbonate  (1. 2  gm.  :  100  c.c.  H2O) 7  c.c. 

I  Distilled  water 100  c.c. 

R     f  Hematoxylin,  crystals i  gm. 

I  Absolute  alcohol 10  c.c. 

e.  Stain  sections  for  12  to  24  hours  in  mixture  composed  of  9 
volumes  of  A  +  1  volume  of  B. 

f.  Wash  thoroughly  in  distilled  water. 

g.  Transfer  to  90  per  cent. — 15  minutes. 
h.  Transfer  to  95  per  cent. — 15  minutes. 

i.  Transfer  to  anilin  oil-xylol  (anilin  oil  2  vol.  -f  xylol  1  vol.) — 5 
minutes. 

j.  Transfer  to  pure  xylol — 5  minutes. 

k.  Mount  in  balsam. 

The  exact  degree  of  color  must  be  determined  by  experience  and 
the  individual  taste  of  the  worker  ;  the  Weigert  method  in  any  of 
its  forms  is  a  stain  particularly  for  the  medullated  nerve-fibres,  the 
cellular  elements  being  better  displayed  by  carmine  or  other  haema- 
toxylin  dyes. 

Golgi's  silver  method  for  displaying  the  nerve-cells  of  the  cen- 
tral nervous  system  has  attracted  much  attention  during  the  last  few 
years,  and  has  very  materially  extended  our  knowledge  of  the  rela- 
tions of  these  elements.  The  method  is  one  of  impregnation  and 
subsequent  reduction,  and,  unfortunately,  in  addition  to  being  un- 
certain, yields  preparations  prone  to  change.  As  it  at  present  plays 
a  conspicuous  rdle  in  the  investigation  of  important  organs,  it  is  here 
given  as  now  suggested  by  Golgi,  the  method  as  used  by  Ramon  y 
Cajal  being  slightly  modified. 

a.  Harden  very  small  pieces  of  the  tissues  in — 

Solution  of  potassium  bichromate  (2  per  cent.) 80  c.c. 

Solution  of  osmic  acid  (1  per  cent.) 10  c.c. 

b.  Transfer  directly  to — 

Silver  nitrate .75  gm. 

Water,  distilled 100  c.c. 

for  24  hours. 

c.  Transfer  to  95  per  cent,  alcohol — 12-24  hours. 

d.  Cut  sections. 


APPENDIX.  427 

e.  Wash  in  80  per  cent,  alcohol — %—i  hour. 

f.  Dehydrate  in  absolute  alcohol. 

g.  Clear  in  turpentine  oil,  or  first  in  clove  oil. 
h.  Balsam  or  damar. 

In  case  the  sections  are  not  sufficiently  dark  after  cutting,  the  pre- 
cipitate can  be  blackened  by  treatment  with  solution  of  sodium 
sulphate. 

Golgi's  gold  method  is  useful  for  displaying  naked  axis-cylinders 
and  ultimate  nerve-fibrillae,  as  well  as  special  nerve-endings  :  the 
method  possesses  the  advantage  of  being  relatively  certain  and  rapid 
in  its  action,  especially  if  the  reduction  be  facilitated  by  heat. 

Soak  the  fresh  tissue  in — 

a.  Arsenious  acid .5  gm. 

Water,  distilled 100  c.c. 

until  it  becomes  translucent — usually  15-25  minutes. 

b.  Transfer  to — 

Gold  chloride .5  gm. 

Water,  distilled 100  c.c. 

for  25  to  45  minutes  ;  rinse  off  in  distilled  water. 

c.  Transfer  to  1  per  cent,  arsenious  acid  solution  and  expose  to 
sunlight  until  reduction  follows  and  the  tissue  appears  of  a  deep 
purple  or  red  color  ;  this  reduction  may  be  hastened  by  gently  heat- 
ing over  water-bath  for  some  10  to  15  minutes,  until  the  tissue  be- 
comes deeply  colored. 

d.  Wash  thoroughly  in  water. 

e.  Transfer  to  alcohol  for  dehydration,  or  to  50  per  cent,  glycerin, 
as  the  case  may  demand  respectively  for  balsam  or  glycerin  mounting. 

Silver  staining  is  an  important  means  of  bringing  to  view  the 
boundaries  of  epithelial  and  endothelial  cells  by  the  deposit  of  reduced 
silver  particles  within  the  intercellular  cement-substance  ;  in  the  typical 
silver  staining  only  the  cell  boundaries  are  shown  as  dark  brown  or 
black  lines,  the  protoplasm  being  almost  colorless.  In  intensely 
stained  specimens  of  very  fresh  still  living  tissue  the  protoplasm  and 
nuclei  are  sometimes  colored.  The  silver  method  also  tinges  the 
interfibrillar  ground-substance  of  dense  connective  tissue,  bringing 
to  view  the  cell-spaces  as  clear  areas  within  a  colored  field. 

The  absolutely  fresh  tissue  is  carefully  rinsed  in  distilled  water, 
without  rubbing  the  surfaces,  and  then  transferred  to  .5-1  per  cent, 
solution  of  silver  nitrate  from  2  to  10  minutes,  depending  on  the 
thickness  of  the  object ;  the  then  milky  tissue  is  washed  and  exposed 


49g  APPENDIX. 

in  distilled  water  to  sunlight  in  a  porcelain  dish  until  it  becomes  dark 
brown  ;  the  reduction  is  arrested,  when  sufficiently  advanced,  by- 
thorough  washing  in  water  to  which  a  few  grains  of  sodium  chloride 
have  been  added.  The  stained  tissue  may  be  mounted  either  in 
glycerin  or  in  balsam,  soaking  in  dilute  and  later  strong  glycerin, 
or  dehydration  and  clearing,  being  the  subsequent  respective  manipu- 
lations. 

Staining  chromatin  filaments  for  the  display  of  karyokinetic 
figures  and  other  studies  of  cell-structure  can  be  successfully  carried 
out  only  after  accurate  fixation  of  the  cells,  for  which  purpose  the 
stronger  Flemming's  solution  will  be  found  most  trustworthy. 

The  tissue  after  such  treatment  is  embedded  in  paraffin  and  cut, 
the  fixed  sections  on  the  slide  being  subsequently  stained  by  saffranin 
or  by  Delafield's  hsematoxylin.  When  karyomitosis  is  the  especial 
object  of  study,  preparations  made  by  stripping  off  the  epidermis  of 
suitable  animals  {very  young  larval  newts  being  excellent)  are  more 
favorable  than  sections,  as  the  cells  are  preserved  intact  and  contain 
the  entire  chromatin  figures,  and  not  merely  the  parts  included  within 
the  planes  of  the  section.     Place  small  pieces  of  such  tissues  in — 

a.  Saffranin 2  gm. 

Alcohol,  50  per  cent 60  c.c. 

24-48  hours. 

b.  Wash  off  in  water  for  a  few  moments. 

c.  Transfer  to  acidulated  absolute  alcohol  (10  drops  of  pure  hydro- 
chloric acid  to  100  c.c.  of  absolute  alcohol)  for  a  few  moments  (j4—i 
minute)  until  the  clouds  of  color  cease  to  be  copiously  given  off; 
then — 

d.  Transfer  to  fresh  absolute  alcohol  for  1  to  2  minutes. 

e.  Clear  in  clove  oil  and  mount. 

Care  must  be  taken  not  to  remove  too  much  color  by  prolonged 
action  of  either  the  acidulated  or  the  plain  absolute  alcohol,  since 
the  preparation  can  be  almost  entirely  bleached  by  inattention  to 
this  point.  In  a  successful  preparation  the  chromatin  figures  are 
brilliantly  stained  of  a  bright  red,  while  the  other  parts  of  the  cells 
are  almost  uncolored. 

Injection  of  capillary  blood-vessels  requires  considerable  ex- 
perience, and  at  best  an  element  of  uncertainty  enters  into  every 
attempt,  since  the  condition  of  the  tissues,  particularly  of  the  vessels, 
largely  influences  the  manner  in  which  the  fluid  runs.  While  car- 
mine-gelatin injections  make  very  attractive  pictures,  a  successful 
blue  mass  possesses  many  advantages  when  used  in  connection  with 


APPENDIX. 


429 


carmine  solutions.  One  of  the  most  convenient  and  efficient  inject- 
ing fluids  is — 

Soluble  Berlin  blue  ( Griibler) 3  gm. 

Distilled  water 600  c.c. 

This  fluid  runs  well,  does  not  extravasate,  and  may  be  used  cold ; 
perfectly  fresh  animals,  immediately  after  killing,  are  the  most  favor- 
able subjects  for  the  manipulation.  A  smoothly-working  hand- 
syringe  (200—300  c.c.  capacity),  with  appropriate  stop-cock  and  can- 
ulae,  is  the  best  instrument,  since  the  educated  hand  of  the  operator 
forms  the  best  judge  of  the  amount  of  pressure  that  may  safely  be 
applied.  When  the  injection  is  completed  the  vessels  should  be 
ligated  and  the  tissue  placed  in  70  per  cent,  alcohol  or  Muller's  fluid 
for  fixation  and  subsequent  hardening.  In  the  case  of  the  lungs, 
after  injecting  the  blood-vessels,  the  tissue  should  be  moderately  dis- 
tended by  forcing  the  preserving  fluid  into  the  organ  through  the 
air-tubes. 

In  conclusion,  it  may  be  repeated  that  the  object  in  appending 
these  pages  treating  of  microscopic  technology  is  to  present  in  detail 
a  few  methods  which  will  be  found  satisfactory  and  thoroughly  trust- 
worthy for  the  great  majority  of  histological  investigations.  The 
student  is  urged  to  persevere  with  those  here  given  until  he  has  re- 
peatedly carried  the  manipulations  to  a  successful  issue  by  producing 
the  really  beautiful  results  of  which  these  methods  are  capable. 


INDEX. 


A. 

Absolute  alcohol,  use  of,  4°9- 
Accessory  digestive  glands,  182. 

development  of,  189. 
Acervulus  cerebri,  330. 
Achromatin,  13. 
Adenoid  tissue,  118. 
Adipose  tissue,  43. 
Agminated  glands,  173. 
Alum-haematoxylin  (Bbhmer),  414- 
Amoeboid  movement,  12 
Amphiuma,  red  blood-cells  of.  108. 
Aqueduct  of  Sylvius,  nuclei  of  floor  of,  304. 
Arachnoid,  283. 
villi  of,  283. 
Areolar  tissue,  40. 
Arrector  pili,  272. 
Arterial  glands,  114. 
Arteries,  94. 

adventitia  of,  96. 
intima  of,  95. 
media  of,  95. 
small,  97. 
structure  of,  94. 
Association  fibres  of  cerebrum,  327. 
Attraction-spheres,  14. 
Auerbach,  plexus  of,  73. 
Axis-cylinder,  73. 
processes,  70. 

B. 

Baillarger's  stripes,  311. 
Dartholin's  glands,  237. 
Basement-membranes,  137. 
Berlin-blue  injecting,  429. 
Blastoderm,  23. 
Blastodermic  layers,  24,  25. 
Blood,  105. 

elementary  particles,  in. 

fibrin,  in. 

granules  of  Max  Schultze,  in. 

haematoblasts,  in. 
Blood-cells,  colored,  106. 

colored,  human,  107. 

colorless,  105. 

division  of,  112. 

effect  of  reagents  upon,  108. 

origin  of  colored,  112. 

origin  of  colorless,  112. 

primary  embryonal,  113. 

reproduction  of  colorless,  106. 

size  of,  108. 
Blood-crystals,  in. 

-islands  of  Pander,  103 


Blood-platelets,  no. 
Blood-vessels,  94. 

capillary,  99. 

development  of,  103. 

lymphatics  of,  100. 

nerves  of,  100 

perilymphatic  clefts,  100. 

Bone,  47. 

circumferential  lamellae,  48. 
compact,  47. 
development  of,  51. 
endochondral  formation  of,  52. 
ground  lamellae,  48. 
Haversian  canals,  48. 
Haversian  spaces,  54. 
Howship's  lacunae,  56. 
interstitial  lamella;,  48. 
marrow,  50. 
marrow-cavity,  48. 
osteoblasts,  55. 
osteoclasts,  56. 

perforating  fibres  of  Sharpey,  50. 
periosteal  formation  of,  54- 
periosteum,  49. 
red  marrow,  50. 
spongy,  47. 

summary  of  development  of,  5°- 
varieties  of,  47- 
Borax-carmine  (Grenacher),  412. 
Brain-sand,  283,  330. 
Bronchial  tubes,  250. 
Brunner's  glands,  171. 


Canal  of  Schlemm,  351. 

Capillary  blood-vessels,  99. 

Carmine  staining,  advantages  of,  4"- 

Carotid  gland,  114. 

Cartilage,  44. 

cells,  45. 

development  of,  47. 

elastic,  46. 

fibrous,  47. 

hyaline,  44. 

of  bronchial  tubes,  250. 

of  Santorini,  248. 

of  trachea,  249. 

of  Wrisberg,  248. 

perichondrium  of,  46. 

varieties  of,  44. 
Cells,  embry°nal>  "■ 

goblet,  31. 

granule,  37. 


431 


432 


INDEX. 


Cells,  irritability  of,  21. 

motion  of,  21. 

pigment,  37. 

plasma,  36 

typical,  12. 

wandering,  36. 
Cell-division,  direct,  15. 

indirect,  16. 
Cell-wall,  12. 
Celloidin  method,  417. 
Central  nervous  system,  282. 
Centrosome,  14,  19. 
Cerebellum,  304. 

cells  of  Purkinje,  308. 

granule  layer,  305. 

molecular  layer,  309. 

nuclei  of  the  roof,  310. 

white  matter  of,  310. 
Cerebral  cortex,  blood-vessels  of,  319. 

pyramidal  cells  of,  312,  313. 

stratification  of,  311. 
Cerebrum,  311. 

association  fibres  of,  327. 

claustrum,  320. 

commissural  fibres  of,  327. 

cornu  Ammonis,  312. 

corpus  striatum,  319. 

fascia  dentata,  318. 

fibre-tracts  of,  326. 

fifth  ventricle,  319. 

fimbria,  318. 

hippocampus  major,  315. 

nerve-fibres  of,  314. 

nucleus  caudatus,  319. 

nucleus  lenticularis,  320. 

optic  thalamus,  320. 

peduncles  of,  303. 

projection  fibres  of,  327. 

septum  lucidum,  319. 

white  matter  of,  326. 
Ceruminous  glands,  275. 
Charcot's  prostatic  crystals,  222. 
Choroidal  fissure  of  eye,  373. 
Chromatin,  13. 

figures,  staining  of,  428. 
Ciliary  motion,  30. 

effects  of  reagents  upon,  31. 
Claustrum,  320. 
Clitoris,  237. 
Coccygeal  gland,  114. 
Ccelom,  133. 
Colostrum,  242. 
Conarium,  329. 
Connective  tissue,  35. 

arrangement  of  cells  of,  38. 

cellular  elements  of,  36. 

development  of,  42. 

ground-substance  of,  40. 

juice-canals  of,  39. 

migratory  cells  of,  36. 

mucoid,  40. 

spaces  of,  39. 

varieties  of,  35. 

wandering  cells  of,  36. 

white  fibrous,  39. 


Connective  tissue,  yellow  elastic,  39. 

Conus  medullaris,  284. 

Cornea,  336. 

Corneal  corpuscles,  338. 

Corpora  amylacea  of  brain,  330. 

geniculata,  322. 

mammillaria,  323. 

quadrigemina,  321. 
Corpus  callosum,  327. 

striatum,  319. 

subthalamicum,  321. 
Cowper's  glands,  222 
Crescents  of  Gianuzzi,  141. 
Crura  cerebri,  303. 

crusta  of,  303,  304 

fibres  of  crusta,  327. 

substantia  nigra,  304. 

tegmentum  of,  303,  304. 
Cutis  anserina    272. 

D. 

Debove's  endothelium  of  intestine,  169 
Decidua,  uterine,  233. 
Delafield's  hematoxylin,  413. 
Demilunes  of  Heidenhain,  141. 
Dentine,  51. 

Derivatives  of  blastodermic  L.yers,  25. 
Direct  cell-division,  15. 
Duct,  endolymphatic,  396. 

galactophorous,  240. 

Gartner's,  230,  245. 

Miiller's,  242. 

of  thyroid  body,  257. 

Wolffian,  204,  242. 
Dura  mater,  282. 

nerves  of,  283. 

perivascular  lymphatics  of,  282. 

venous  sinus  of,  282. 


Ear,  377. 

accessory  spiral  ligament,  390 

ampulla  of  semicircular  canals,  387 

auditory  pit,  398. 

auditory  teeth,  391. 

basilar  membrane,  391. 

canalis  reuniens,  383. 

cells  of  Claudius,  394,  395. 

cells  of  Deiters,  394,  395. 

cells  of  Hensen,  394,  395. 

ceruminous  glands  of,  377. 

cochlea,  388. 

cochlea,  blood-vessels  of,  397. 

cochlea,  nerves  of,  395.      « 

cochlea,  perilymph-spaces  of,  396. 

Corti's  organ,  392. 

crista  basilaris,  389. 

crista?  acusticae  of  semicircular  canals,  386. 

development  of,  397. 

ductus  cochlearis.  388,  389. 

ductus  endolymphaticus,  383.  396. 

Eustachian  tube,  382. 

external  auditory  canal,  377. 

external  cartilage  of,  377. 

fenestra  ovalis,  382. 


INDEX. 


433 


Ear,  hair-cells,  394. 

hair-cells  ol  maculae  acusticse,  385. 

internal,  383. 

ligamentum  spirale,  389. 

maculse  acusticse  of,  384. 

mastoid  cells,  epithelium  of,  380. 

membrana  tectoria,  395. 

membrane  of  Reissner,  389. 

middle,  epithelium  of,  380 

middle,  glands  of,  380. 

ossicles,  381,  382. 

otic  vesicle,  398. 

otolith-membrane,  385. 

otoliths,  385. 

pillars  of  Cord,  392. 

prominentia  spiralis,  390. 

saccule,  383. 

saccus  endolymphaiicus,  396. 

secondary  tympanic  membrane,  38 

semicircular  canals,  386. 

spaces  of  Nuel,  395. 

spiral  lamina,  388. 

stria  vascularis,  390. 

sulcus  spiralis,  391. 

tympanic  cavity,  380. 

tympanic  membrane,  378. 

tympanum,  lymphatics  of,  379. 

tympanum,  nerves  of,  379. 

utricle,  383. 
Ear-stones,  385. 
Ectoderm,  24. 

derivatives  of,  25. 
Elastic  tissue,  42. 
Elastin,  40. 
E'eidin,  272. 
Elementary  tissues,  11. 
Embedding,  interstitial,  414. 

simple,  414. 
Embryonal  cell,  11. 
Endochondral  formation  of  bone,  52. 
Endogenous  cell-division,  20. 
Endothelium,  33. 

development  of,  34. 

stomata  of,  33. 
Entoderm,  24. 

derivatives  of,  25. 
Epididymis,  213. 

development  of,  243. 

globus  major  of,  208,  212. 

tube  of,  213. 
Epidural  spaces,  282. 
Epiglottis,  248. 

glands  of,  248. 
Epiphysis,  329. 
Episcleral  space,  372. 
Epithelium,  26. 

ciliated,  30. 

classification  of,  26 

columnar,  29 

development  of,  27,  34. 

distribution  of,  27. 

germinal,  of  ovary,  224. 

glandular,  31. 

modified,  30. 

of  mucous  membranes,  136. 


28 


Epithelium  of  sense-organs,  32. 

pigmented,  31. 

prickle-cells,  29. 

rod,  32. 

squamous,  27. 

transitional,  29. 

varieties  of,  26. 
Eponychium,  278. 
Epoophoron,  230. 
Equatorial  plate,  17. 
Erythroblasts,  113. 
Eustachian  tube,  382. 
Eye,  336. 

anterior  chamber  of,  366. 

blood-vessels  of,  365. 

canal  of  Petit,  366. 

canal  of  Schlemm,  351 

capsule  of  Tenon,  366,  372. 

choriocapillaris,  343. 

choroid,  342. 

choroidal  fissure,  373. 

ciliary  body,  344. 

ciliary  muscle,  345. 

ciliary  processes,  344. 

color  of  iris,  349. 

conjunctiva,  369. 

cornea,  336. 

crystalline  lens,  361. 

development  of,  372. 

fovea  centralis,  356. 

hyaloid  canal,  365. 

hyaloid  membrane,  364. 

irido-corneal  angle,  349. 

iris,  346. 

iris,  color  of,  349. 

lachrymal  canals,  371. 

lachrymal  caruncle,  370. 

lachrymal  gland,  371. 

lamina  cribrosa,  360. 

lamina  fusca,  341. 

lamina  suprachoroidea,  341. 

ligamentum  pectinatum  iridis,  350. 

lymphatics  of,  366. 

macula  lutea,  356. 

membrana  nictitans,  370. 

naso-lachrymal  duct,  371. 

nerves  of,  367. 

optic  nerve,  358. 

optic  nerve,  excavation  of,  360. 

optic  nerve,  sheaths  of,  359. 

optic  vesicle,  372. 

ora  serrata,  357. 

perichoroidal  space,  366. 

plica  semilunaris,  370. 

retina,  351. 

retina,  morphology  of,  351. 

retina,  pigment-layer  of,  356. 

retina,  visual  cells  of,  354. 

sclera,  341. 

scaces  of  Fontana,  350 

suspensory  ligament  of  lens,  363. 

tapetum  cellulosum,  343. 

tapetum  fibrosum,  343. 

Tenon's  space,  366. 

venae  vorticosae,  343,  366, 


434 

Eye,  vitreous  body,  364. 

vitreous  lamina,  344. 

zone  of  Zinn,  363. 
Eyelids,  367. 

blocd-vessels  of,  370. 

development  of,  376 

glands  of  Moll,  370 

lymphatics,  370. 

Meibomian  glands,  369. 

nerves  of,  371. 

tarsus,  368. 

F. 
Fallopian  tube,  230. 
Fat,  43. 
Fat-cells,  43. 
Female  pronucleus,  22. 
Fenestrated  membrane  of  Henle,  95. 
Finishing  and  storing  preparations,  423. 
Fixation  of  tissues,  408. 
Fixing  sections  to  the  slide,  420. 
Flemming's  solution,  409. 
Foramen  caecum,  257. 


Galactophorous  ducts,  240. 
Gall-bladder,  182. 
Ganglia,  structure  of,  80. 
Gartner's  duct,  230. 
Genitalia,  female,  236 
Germ-cells  of  neural  tube,  333. 
Germinal  epithelium,  224. 
spot,  227. 
vesicle,  227. 
Giraldes's  organ,  214. 
Glands,  137. 

arterial,  114. 

Bartholin's,  237. 
blood-vessels  of,  141. 

Brunner's,  171. 

carotid,  114. 

coccygeal,  114. 

compound  saccular,  138. 

compound  tubular,  137. 

Cowper's,  222. 

development  of,  142. 

epithelium  of,  32. 

Harder's,  370. 

lachrymal,  371. 

Lieberkiihn's,  170. 

IJttre's,  203. 

lymphatics  of,  142. 

Moll's,  370. 

Montgomery's,  240. 

mucous,  140. 

Naboth's,  233. 

nerves  of,  142. 

racemose,  138. 

secreting  cells  of,  141. 

serous,  140. 

simple  saccular,  138. 

simple  tubular,  137. 

solitary,  172. 

structure  of,  138. 

Tyson's,  219. 

unicellular,  137. 


INDEX. 


Glands,  varieties  of,  137. 
Golgi's  gold  method,  427. 

silver  method,  426. 
Graafian  follicles,  226. 

number  of,  228. 
Grandry's  tactile  corpuscles,  84. 
Granule-cells,  37. 
Growth,  15. 

H. 

Haematoxylin  staining,  advantages  of,  411. 

Weigert's  method,  425. 
Hair,  268. 

color  of  269. 

development  of,  279. 

lanugo,  280. 

race  peculiarities  of,  267, 

renewal  of,  280 

structure  of,  268 
Hair-follicles,  269. 

root-sheaths  of,  270. 

structure  of,  271. 
Hassall's  corpuscles  of  thymus.  128. 
Haversian  canals,  4S. 
Heart,  100. 

annuli  fibrosi,  101. 

blood-vessels  of,  102. 

chordae  tendineae,  101. 

corpus  Arantii,  101. 

development  of,  104. 

endocardium,  100. 

fibres  of  Purkinje,  101. 

lymphatics  of,  103. 

muscular  tissue  of,  102. 

nerves  of,  103. 

pericardium,  102. 

structure  of,  100. 

valves  of,  100. 
Howship's  lacunae,  56. 
Hyaloplasm,  13. 
Hydatid,  sessile,  244 

stalked,  of  Morgagni,  230 
Hypophysis  cerebri,  328. 

I. 

Indirect  cell-division,  16. 
Injecting  capillary  blood-vessels,  429. 
Internal  capsule,  327. 
Intestines,  168. 

agminated  glands,  173. 

blood-vessels  of,  175. 

Brunner's  glands  of,  171. 

chyle  vessels  of,  169. 

duodenal  glands  of,  171. 

glands  of,  170. 

goblet-cells  of,  169. 

Lieberkiihn's  follicles  of,  170. 

lymphatics  of,  175. 

mucosa  of  large,  169. 

mucosa  of  small,  168. 

muscular  coat  of,  174. 

muscular  coat  of  large,  174. 

muscularis  mucosae  of,  174. 

nerves  of,  176. 

Peyer's  patches,  173 

solitary  glands  of,  172. 


INDEX. 


435 


Intestines,  submucosa  of,  174. 
valvulae  conniventes,  168. 
villi  of,  168,  169. 

K. 

Karyokinesis,  15. 
Kidney,  191. 

blood-vessels  of,  199. 

Bowman's  capsule,  193. 

capsule  of,  193. 

columns  of  Bertini,  192. 

connective  tissue  of,  193. 

development  of,  204. 

divisions  of,  191. 

glomeruli  of,  193. 

Henle's  loops,  195. 

labyrinth  of,  192. 

lobules  of,  191. 

lymphatics  of,  200. 

Malpighian  bodies,  193. 

Malpighian  pyramids,  192. 

Malpighian  tuft,  193. 

medulla  of,  192. 

medullary  rays,  192. 

nerves  of,  200. 

papillae  of,  191. 

pelvis  of,  201 

sinus  of,  201. 

tubes  of  Bellini,  195. 

uriniferous  tubules  of,  195,  196. 
Kleinenberg's  solution,  410. 
Krause's  views  regarding  muscle,  63. 

L. 

Labia  majora,  236. 

minora,  236. 
Lachrymal  canals,  371. 

gland,  371. 
Lanugo,  280. 
Larynx,  246. 

blood-vessels  of,  248. 

cartilages  of,  248. 

lymphatics  of,  248. 

nerves  of,  248. 

vocal  cords  of,  246. 
Leucocytes,  105. 
Lieberkiihn's  follicles,  170. 
Ligamentum  nuchas,  42. 
Liver,  176. 

bile-capillaries  of,  178,  179. 

bile-ducts  of,  180. 

blood-vessels  of,  181. 

cells  of,  178. 

development  of,  189. 

fibrous  tissue  of,  176. 

glands  of  bile-ducts,  181. 

Glisson's  capsule,  176. 

interlobular  vessels  of,  177. 

intralobular  capillary  net-work,  177. 

lymphatics  of,  181. 

multinucleated  cells  of,  189. 

nerves  of,  182. 

perivascular  lymphatics  of,  179. 
Lung,  252. 

air-sacs,  252. 


Lung,  alveolar  passages,  251. 

blood- vessels  of,  254. 

connective  tissue  of,  254. 

development,  259. 

infundibula,  251. 

lobules  of,  252. 

lymphatics  of,  255. 

nerves  of,  255. 

pigment  within,  254. 

terminal  bronchus,  251. 
Lunula  of  nail,  265. 
Luschka's  gland,  114. 
Lymph,  117. 

capillaries,  116. 

corpuscles,  117. 

corpuscles,  sources  of,  118. 

perineurial  channels,  117 

perivascular  sheaths,  117 
Lymphatic  glands,  119. 

blood-vessels  of,  120 
compound,  120. 
simple,  119. 

spaces,  115. 

system,  115. 

system,  development  of,  133. 

tissues,  118. 

tissues,  diffuse,  119. 

tissues,  elements  of,  118. 

vessels,  117. 

M. 

Male  pronucleus,  23. 
Mammary  glands,  238. 

ampullae,  240. 

areola,  240. 

blood-vessels,  241. 

during  lactation,  239 

galactophorous  ducts,  ^40 

lymphatics,  241. 

nerves,  241. 

nipple,  240. 

rudimentary,  241. 
Maturation  of  ovum,  22. 
Medulla,  295. 

anterior  pyramid,  300. 

arcuate  fibres,  298,  299. 

clavus,  296. 

corpus  dentatum  of  olive,  299. 

cuneate  tubercle.  296. 

decussation  of  anterior  pyramids,  297 

external  cuneate  nucleus,  297. 

fibre-tracts  of,  300. 

formatio  reticularis,  298 

funiculus  Rolandi,  296. 

funiculus  teres,  297. 

hypoglossal  nucleus,  297. 

lateral  tract,  300. 

nucleus  cuneatus,  296. 

nucleus  gracilis,  296. 

nucleus  lateralis,  297. 

posterior  pyramid,  301. 

restiform  body,  300 

spinalis,  284. 
Meissner's  tactile  corpuscles,  85 
Melanin,  38. 


436 


INDEX. 


Membrana  nictitans,  370. 

propria  of  mucous  membranes,  137. 
Membrane  of  Descemet,  339. 
Merkel's  tactile  corpuscles,  84. 
Mesoderm,  24. 

derivatives  of,  25. 
Mesogastrium,  190. 
Mesothelium,  133. 
Metabolism,  15. 
Metakinesis,  18. 
Microcytes  of  blood,  in 
Milk,  241 

colostrum  corpuscles,  242. 

secretion  of,  239. 
Mitotic  cell-division,  16. 
Mounting  sections,  422. 
Mouth,  144. 

blood-vessels  of,  145 

lymphatics  of,  145. 

mucous  membrane  of,  144 

nerves  of,  145. 
Mucous  membranes,  structure  of,  136 
Muscle,  58. 

blood-vessels  of,  67. 

cardiac,  66. 

development  of,  67. 

involuntary,  59. 

nerves  of,  67. 

non-striped,  distribution  of,  58. 

striped,  61. 

voluntary,  61. 
Miillerian  duct,  242. 
Miiller's  fibres  of  retina,  352. 
Muller's  fluid,  408. 
Myeloplaxes  of  Robin,  51. 

N. 

Nails,  265. 

development  of,  278. 

growth  of,  278. 

lunula,  279. 

regeneration  of,  279. 

structure  of,  266. 
Nasal  mucous  membrane,  402. 

blood-vessels  of,  404. 

Bowman's  glands,  404 

development  of,  405 

glands  of,  402. 

lymphatics  of,  405. 

nerves  of,  405. 

olfactory  division  of,  403. 

olfactory  epithelium,  404. 

respiratory  division  of,  402. 
Nasmyth's  membrane,  148. 
Naso-lachrymal  duct,  371. 
Nebenkern,  14. 
Nephrostomata,  197. 
Nerve-cells,  69. 

of  first  type,  70. 

of  second  type,  71. 

processes  of,  70. 
Nerve-endings,  83. 

classification  of,  88. 

cylindrical  end-bulbs,  86. 

in  blood-vessels,  92. 


Nerve-endings  in  glands,  92. 

in  non-striated  muscle,  89. 

in  striated  muscle,  90. 

muscle-spindles,  91. 

of  Langerhans,  84. 

sensory,  83. 

spherical  end-bulbs,  85. 

tactile  cells,  84. 

tendon  spindles,  92. 
Nerve-fibres,  72. 

medullated,  73 

non-medullated,  74. 

of  spinal  cord,  288. 
Nerve-trunks,  77. 

blood-vessels  of,  78. 

endoneurium,  77. 

epineurium,  78. 

funiculus,  77. 

Henle's  sheath  of,  78. 

lymphatics  of,  78. 

nerves  of,  78. 

perineurium,  77. 
Nervi  nervorum,  78. 
Nervous  system,  282. 
Nervous  tissues,  69. 

development  of,  81. 

supporting  framework  01,  79. 
Neuroblasts,  81,  333. 
Neuro-epithelium,  93. 
Neuroglia,  79. 

cells,  79. 
Nipple,  240. 
Nucleolus,  14. 
Nucleus,  13. 

caudatus,  319. 

fibrils  of,  13. 

lenticularis,  320. 

membrane  of,  13. 

segmentation,  23. 

structure  of,  13. 
Nymphs,  236. 


CEsophagus,  160. 

muscular  tissue  of,  161. 
Olfactory  bulb,  324. 

glomeruli,  325. 

lobe,  323. 

tract,  323. 
Optic  thalamus,  320. 
Organ  of  Rosenmuller,  230. 
Ovary,  224. 

blood-vessels,  229. 

corpus  luteum  of  pregnancy,  229. 

development  of,  244. 

germinal  epithelium,  224. 

interstitial  cells,  228. 

lymphatics  of,  229. 

nerves  of,  229. 

primary  egg-tubes,  244. 

primordial  ova,  244. 

stroma,  225. 

tunica  albuginea,  225. 
Oviduct,  230. 
Ovula  Nabothi,  233. 


INDEX. 


437 


Ovum,  227. 

escape  of,  228. 
germinal  spot,  227. 
germinal  vesicle,  227. 
maturation,  22. 
segmentation  of,  23. 

P. 

Pacchionian  bodies,  283. 
Pacinian  corpuscles,  86. 
Pancreas,  185. 

development  of,  189. 
Panniculus  adiposus,  265. 
Paradidymis,  214. 

development  of,  243. 
Paranucleus,  14. 
Parasinoidal  spaces,  282. 
Paroophoron,  245. 

development  of,  245. 
Parovarium,  230. 

development  of,  244. 
Peduncular  fibres,  327. 
Penis,  216. 

arteries  of,  218. 

cavernous  venous  channels  of,  217. 

corpora  cavernosa,  216. 

corpus  spongiosum,  218. 

erectile  tissue  of,  218. 

glands  of  Tyson,  219. 

helicine  arteries  of,  218. 

lymphatics  of,  219. 

nerves  of,  219. 
Perichondrium,  45. 
Perionyx,  278. 
Periosteal  bone,  54. 
Peripheral  nerve-endings,  83. 
Peyer's  patches,  173. 
Phagocytes,  106. 
Pharynx,  159. 

glands  of,  159. 

mucous  membrane  of,  159. 
Pia  mater,  283. 

pigment-cells  of,  284. 
Picro-sulphuric  acid  solution,  410. 
Pigment-cells,  37. 
Pigment  of  hair,  269. 

of  skin,  263. 
Pineal  body,  329. 

eye,  329. 
Pituitary  body,  328. 
Plasma-cells  of  Waldeyer,  36. 
Plastin,  13. 
Pleura,  256. 

blood-vessels  of,  256. 

nerves  of,  256. 
Plexus  of  Auerbach,  167. 

of  Meissner,  167. 
Polar  bodies,  22. 

field,  16. 
Pole-corpuscle,  14. 
Pons,  301. 

locus  coeruleus,  302. 

nuclei  of,  301. 

posterior  longitudinal  bundle,  302. 

substantia  ferruginea,  302. 


Preparation  of  tissues,  order  of  manipulations, 

407. 
Preservation  of  tissues,  410. 
Primary  body-cavity,  133 

neural  tube,  332. 
Primordial  ova,  244. 

sexual  cells,  243. 
Projection  fibres,  327. 
Pronucleus,  female,  22 

male,  23. 
Prostate  gland,  219. 

acini  of,  220. 

amyloid  concretions  of,  216. 

blood-vessels  of,  222. 

concretions  of,  222. 

involuntary  muscle  of,  220. 

lymphatics  of,  222. 

nerves  of,  222. 

secretion  of,  222. 
Prostatic  crystals,  216. 

sinus,  221. 
Protoplasm,  reticulation  of,  13. 

structure  of,  12. 
Purkinje's  ganglion-cells,  308. 

basket-works  of,  310. 

R. 
Reissner's  membrane,  389. 
Reproduction  of  the  cell,  15. 
Reproductive  organs,  table  of  homologies  of,  245. 
Respiratory  organs,  246. 
Rete  Malpighii,  262. 
Ribbon  sections,  420. 
Rollett's  views  regarding  muscle,  63. 
Root-sheaths  of  hair-follicle,  271 


Saffranin  staining,  428. 
Salivary  corpuscles,  156. 
Sebaceous  glands,  273. 
Sebum,  273. 
Section-cutting,  418. 
Segmentation  nucleus,  23. 
Segmentation  of  ovum,  23. 
Semen,  273. 
Serous  membranes,  128 

blood-vessels  of,  130. 

classification  of,  128. 

development  of,  133. 

ground-substance  of,  130. 

nerves  of,  131. 

structure  of,  129. 
Sexual  cords,  243. 

glands,  indifferent,  243. 
Sharpey's  fibres  of  bone,  50. 
Silver  staining,  427 
Sinus  pocularis,  221,  244. 
Skin,  261. 

arrector  pili,  272. 

blood-vessels  of,  276. 

corium,  264. 

development  of,  277. 

eleidin  granules,  263. 

epidermis,  262. 

epitrichial  layer,  263. 


43§ 


INDEX. 


Skin,  hair-follicles,  269. 

hair  papillae,  272. 

lymphatics  of,  276. 

muscles  of,  272. 

nerves  of,  277. 

panniculus  adiposus,  265. 

papillse  of,  264. 

pigment  of,  263. 

sebaceous  glands,  273. 

stratum  corneum,  263. 

stratum  granulosum,  263. 

stratum  lucidum,  263. 

stratum  Maipighii,  263. 

structure  of  hair.-.,  268. 

sweat-glands,  273. 
Somatopleure,  133. 
Spaces  of  Fontana,  350. 
Spermatic  crystals,  216. 

duct,  213. 
Spermatozoa,  215. 

vibrations  of,  215. 
Spinal  cord,  284. 

anterior  column,  285. 

anterior  gray  commissure,  290. 

anterior  ground-bundle,  286. 

anterior  median  fissure,  285. 

anterior  radicular  zone,  286. 

ascending  anterolateral,  286. 

blood-vessels  of,  294. 

Burdach's  column,  286. 

central  canal  of,  293. 

Clarke's  column,  290. 

connective  tissue,  framework  of,  287. 

crossed  pyramidal  tract,  286. 

descending  antero-lateral,  286. 

direct  cerebellar  tract,  286. 

direct  pyramidal  tract,  286. 

ependyma  of,  293. 

filum  terminale,  285. 

ganglion-cells,  290. 

Goll's  column,  286. 

Gowers's  tract,  286. 

gray  commissure,  285. 

gray  matter,  arrangement  of,  288. 

lateral  column,  286. 

mixed  lateral  tract,  286. 

neuroglia  of,  287. 

outlying  ganglion-cells,  291. 

posterior  column,  285. 

posterior  median  fissure,  285. 

structure  of  gray  matter.  290,  291. 

substaniia  gelatinosa,  293. 

substantia  gelatinosa  Rolandi,  293. 

substantia  spongiosa  of,  291. 

Tiirck's  column,  286. 

ventriculus  terminalis,  294. 

white  commissure,  288. 

white  matter  of,  287. 
Spirem,  16. 
Splanchnopleure,  133. 
Spleen,  122. 

blood-vessels  of,  125. 

development  of,  134. 

lymphatics  of,  126. 
Malpighian  corpuscles  of,  124. 


Spleen,  nerves  of,  126. 

pulp-tissue  of,  124. 
Spongioblasts,  81,  333. 
Spongioplasm,  12. 
Staining,  411. 

chromatin  figures,  428. 
Standard  technique,  outline  of,  423. 
Stomach,  162. 

acid  cells  of,  163. 

blood-vessels  of,  166. 

development  of,  187. 

lymphatics  of.  167. 

mucous  membrane  of,  162. 

muscular  coat  of,  166 

nerves  of,  167. 

peptic  glands,  162. 

pyloric  glands  of,  164. 

serous  coat  of,  166. 

submucosa  of,  165 
Stratum  Maipighii,  263. 
Subarachnoidean  space,  283. 
Substantia  gelatinosa  of  spinal  cord,  293. 
Substantia  spongiosa  of  spinal  cord,  291. 
Suprarenal  body,  330. 
Sweat-glands,  273. 

muscle  of,  275. 

number  of,  275. 

secretion  of,  275. 
Synovial  membranes,  131. 

blood-vessels  of,  132. 

Haversian  fringes  of,  132. 

nerves  of,  132. 

structure  of,  131. 


Tapetum  cellulosum  of  choroid,  343. 
Tapetum  fibrosum  of  choroid,  343. 
Taste-buds,  155. 
Teeth,  145. 

cementum,  148. 

crusta  petrosa,  148. 

dental  papilla,  150. 

dentinal  fibres,  147 

dentinal  tubules,  146. 

dentine,  146. 

development  of,  149. 

enamel,  147. 

enamel  organ,  150. 

incremental  lines  of  Salter,  147. 

interglobular  spaces,  146. 

membrane  of  Nasmyth,  148. 

odontoblasts,  152. 

pulp,  148. 

Schrager's  lines,  147. 

stripes  of  Retzius,  148. 
Tegmentum  of  cerebral  peduncles,  32S 
Tendon,  structure  of,  41. 
Tendon-cells,  41. 
Tenon's  capsule,  372. 
Testicle,  207. 

blood-vessels  of,  214. 

coni  vasculosi,  208,  213. 

development  of,  243. 

hydatids  of,  214. 

interstitial  cells,  212. 


INDEX. 


439 


Testicle,  lymphatics  of,  214. 

mediastinum  oi,  207. 

nerves  of,  214. 

seminiferous  tubules,  208. 

Sertoli's  columns,  209. 

spermatoblasts,  209. 

spermatogenesis,  210. 

straight  tubules,  212. 

tunica  albuginea,  207. 

tunica  vaginalis,  207. 

vasa  efferentia,  208,  213. 
Thymus  body,  126. 

blood-vessels  of,  128. 

corpuscles  of  Hassall,  128. 

development  of,  134. 

lymphatics  of,  128. 

nerves  of,  128. 
Thyro-glossal  duct,  257. 
Thyroid  body,  257. 

colloid  secretion  of,  258. 

development  of,  260. 
Tissues,  constituents  of,  11. 

elements  of,  23. 
Tongue,  153. 

blood-vessels  of,  156. 

glands  of,  156. 

lymphatics  of,  156. 

mucous  membrane  of,  152. 

nerves  of,  156. 

papillae  of,  153. 
Tonsils,  158. 
Trachea,  249. 

blood-vessels  of,  250. 

cartilages  of,  249. 

glands  of,  249. 

lymphatics  of,  250. 

nerves  of,  250. 
Tunica  propria  of  mucous  membranes,  136. 

U. 

Ureter,  201. 

development  of,  204. 


Urethra,  203. 

development  of,  206. 

female,  203,  237. 

glands  of,  203,  238. 

male,  203. 
Urinary  bladder,  202. 

development  of,  206. 
Uterus,  232. 

blood-vessels  of,  234. 

cervix,  232. 

development  of,  244. 

menstrual  changes,  233. 

muscular  coat  of,  234. 
Uterus  masculinus,  221,  244. 

V. 

Vagina,  235. 

development  of,  244. 
Vas  deferens,  213. 

ampulla?  of,  213. 
Vasa  vasorum,  99. 
Veins,  97. 

adventitia  of,  98. 

intima  of,  97. 

media  of,  98. 

valves  of,  98. 

variations  in  coats  of,  98. 
Vital  manifestations  of  the  cell,  15. 
Vitelline  membrane,  227. 
Vitellus,  227. 
Vocal  cords,  246. 

W. 

Wandering  cells  of  connective  tissue,  36. 
Weigert's  staining  method,  425. 
White  fibrous  tissue,  39. 
Wolffian  body,  204,  242. 
Wolffian  duct,  204,  242. 
Wolffian  tubules,  242. 


Yellow  elastic  tissue,  39. 


THE    END. 


COLUMBIA   UNIVERSITY 

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